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1) Latter interpolations in '1Thessalonians'

A) 1Th2:14-16 is an obvious post-70C.E., post-gospel and anti-Jewish addition, which Paul could not have written:
"... the Jews [of Jerusalem], who killed the Lord Jesus and the prophets
[as in Mt23:34,37; Lk11:49-50,13:34 "Jerusalem, Jerusalem, that is killing the prophets ..." (YLT)]
' and also drove us out ... They ... are hostile to all men. ... In this way they always heap up their sins to the limit. The wrath of God has come upon them at last.
[reference to the events of 70C.E. (as in Mt21:40-41a) which occurred twenty years after the letter was written!]"
Remarks:
a) Nowhere else did Paul write Jesus was "killed", but in the gospels, the word 'kill(ed)' (Greek root 'apokteino'), concerning Jesus, is used many times, as in GMark 8:31, 9:31, 10:34, 12:7,8 & 14:1.
b) The words 'countrymen' (Greek root 'sumphuletes'), 'drove out' (Greek root 'ekdioko') and 'hostile' (Greek root 'enantios') are never in evidence elsewhere in Paul's (authentic) letters.

Note: both 'apokteino' and 'ekdioko' show in Lk11:49, relative to prophets and apostles: "... I will send to them prophets and apostles, and of these shall they [teachers of the Law] kill and drive out by persecution" (Darby)

B) 1Th1:10 is another addition:
"and to wait for his Son from heaven, whom he raised from the dead -- Jesus, who rescues us from the coming wrath."
Why?
a) 1Th1:10 considerably extends a sentence, when Paul used generally short ones in this letter.
b) 1Th1:10 affirms "he raised [Jesus] from the dead" when later, it is a matter of faith "we believe that Jesus died and rose again" (4:14a)
c) 1Th1:10 is packed with theological tenets, some of them having no counterpart in the letter:
Jesus is Christ (1:1,3,2:6,3:2,4:16,5:9,18,23,28) or/and Lord (1:1,3,2:19,3:11,13,4:1,2,15,16,17,5:2,9,23,28), but never "Son". The expressions:
"God the Father and the Lord Jesus Christ" (1:1)
"our God and Father" (1:3,3:13)
"our God and Father himself and our Lord Jesus" (3:11)
And NOT "... the God and Father of our Lord Jesus Christ ..." (2Co1:3a & Ro15:6),
tend to preclude Paul had then Jesus as the "Son".
Then in this letter, dead are risen (4:14,16) (Greek root 'anistemi'), but never "raised" (root 'egeiro').
d) Paul would not have acknowledged "to wait" as a good thing when he wrote in the same letter:
"And we urge you, brothers, warn those who are idle" (5:14)
Apparently, these "idlers" were a problem in the community:
1Th4:11-12 "Make it your ambition to lead a quiet life, to mind your own business and to work with your hands, just as we told you, so that your daily life may win the respect of outsiders and so that you will not be dependent on anybody."

2) Latter interpolations in 'Hebrews'

A) The last Chapter (13) appears to be an addendum, but may contain a later but authentic (very short) letter (or rather note) by Apollos:
Heb13:1-6,
Heb13:9-16 "... Let us, then, go to him outside the camp, bearing the disgrace he bore [Jesus' crucifixion]. For here we do not have an enduring city [Jerusalem, still existing!], but we are looking for the city that is to come ..."
Heb13:18-20a,
Heb13:21-22 "... Brothers, I urge you to bear with my word of exhortation, for I have written you only a short letter ..."
Heb13:25.
However, additions were inserted in it later on (around 100C.E.):
Heb13:7-8 "Remember your [now deceased!] leaders, who spoke the word of God to you ..."
Heb13:17 "Obey your [new!] leaders and submit to their authority..."
Heb13:20b,24a "... your leaders ..."
No community "leaders" are mentioned in the rest of 'Hebrews'.

B) On the subject of addition, it is rather obvious that Heb9:27-28 is also one of those:
a) The sentence formed by the two verses is abnormally very long (almost 50 words).
b) "Just as a man is destined to die once, and after to face judgment":
Judgment to follow death is never a concept exposed in this letter. Rather, the author emphasized his contemporary faithfuls were living in the "last days" (1:2) and destined to enter "Today" (4:7) God's rest (4:6,11) very soon "in just a very little while" (10:35-39) as "you see the Day approaching" (10:25). Then the Judgment would be applied to only the (alive) sinners and/or defectors (10:26-27,30-31,39).
c) "he [Christ] will appear a second time" seems to have been written to "correct" "he [Christ] has appeared once for all at the end of the ages" (10:26), only two verses earlier.
In 'Hebrews' Christ has no role to play during the "Day of the Lord":
God administers the final Judgment (10:30-31,12:23), delivers the rewards (11:40) and disposes of Christ's enemies (1:13). Meanwhile, "Since that time [the sacrifice on the cross] he [Christ] waits for his enemies to be made his footstool" (10:13).
Then, the Christians, through raptures, are to join Christ, God and angels in the heavenly Jerusalem (12:22-24). No second coming here!
However the "second coming" (on earth) will be prominently featured later on (Mt25:31-32, Ac1:11, Rev19:11-16).

Note: the notion of "second coming" did not start immediately among Gentile Christians. Paul had only a half way second coming of Jesus, down to the clouds below heaven (1Th4:16-17). This is echoed in GMark (13:26-27).

3) Dating of 'Galatians'

The dating of 'Galatians' has been proposed anywhere from 48 to 58C.E. My late dating is based on:

A) Gal2:10 "All they [the "Nazarene" leadership] asked was that we should continue to remember the poor,
[collect "survival" money for the church of Jerusalem: 1Co16:1-4, 2Co8-9, Ro15:25-27]
` the very thing I was eager to do.
[and not "am eager to do" or "will be doing"]"
That would suggest the money collections have been done already, as in late 57 or early 58C.E., or more specifically for the Galatians, in early 55 (1Co16:1).

Note: details on the dating are according to my study, as explained in Appendix B (28) and Paul and the Corinthians

B) According to Gal 4:13-14 "You know that because of physical infirmity I preached the gospel to you at the first. And my trial which was in my flesh you did not despise or reject, ... For I bear you witness that, if possible, you would have plucked out your own eyes and given them to me."
those Galatians got converted because Paul (apparently traveling on his own) had to stay among them in order to recover from illness(es).
That would preclude these Galatians as being inhabitants of the four cities in "southern Galatia" (Antioch Pisidian, Iconium, Lystra & Derbe), the ones visited by Paul (with Barnabas) during the first missionary journey (as narrated in Ac13:14-14:24) and revisited by Paul (with Silas, later joined also by Timothy) in 50C.E. (Ac16:1-6) at the start of the second one. Furthermore, the epistle is not addressed to Christians of a city or to an individual (as for all other Pauline epistles) but rather believers in (a part of) Galatia.
The most likely time for Paul's unplanned stay among (strictly) Gentiles in Galatia is when "... he left [Antioch] and traveled in orderly sequence through the Galatian [Greek root 'galatikos', meaning also "Gallic/Gauls" in ancient Greek literature (Polybius, Appian, Strabo, Plutarch)] country and Phrygia, bringing strength to all the disciples." (Ac18:23 NAB), as "Paul, having passed through the upper districts, came to Ephesus" (Ac19:1 Darby).

Notes:
A) In Ac18:23, according to Strong:
a) The Greek word for "country" is 'chora' and means:
1) the space lying between two places or limits
2) a region or country i.e. a tract of land
. a) the (rural) region surrounding a city or village, the country
. b) the region with towns and villages which surround a metropolis
3) land which is ploughed or cultivated, ground
b) The Greek word for "in orderly sequence" is 'kathexes' and means:
1) one after another, successively, in order
B) Between Ac18:23 and Ac19:1, the following events are recorded in 'Acts': Apollos preaches in Ephesus "in the way of the Lord", then is converted to Pauline Christianity, then goes to Corinth, then keeps preaching over there.
It seems a significant amount of time, like months, elapsed during Paul's journey through Galatia & Phrygia.
C) Phrygia was a large area occupying the eastern half of Asia and some of northern/western (but not "southern") Galatia. Asia and Galatia were Roman provinces at the time, but not Phrygia.
D) Galatia got its name from descendants of Celtic (Gaelic) tribes which migrated in the area around 278B.C.E (about twenty thousand of them). They were known to be, besides former barbaric warriors, pastoral people living in fortified villages. Cities which existed then suffered badly under their occupation. For example, Ancyra, a prosperous city before, was reduced to only a fortresss & village. It will become back a city later, around 150C.E. as related in this Encyclopedic website. Also, under 'Phrygia', from the same site:
"At last Attalus I settled the Gauls permanently in eastern Phrygia, and a large part of the country was henceforth known as Galatia. Strabo [19C.E.] mentions that the great cities of ancient Phrygia were in his time either deserted or marked by mere villages."
Then (according to this other page from the same website) the rural area were populated by some Phrygian remnants and Gauls (Galatians). Because the area was getting hellenized then (1st century), some would know Greek, the universal language of the Near East, more so along the main trade routes.
E) Also against the 'southern Galatia theory' is the fact "Luke" never used "Galatia(ns)" during the narration of Paul & Barnabas first missionary journey. Why?
Even if the area was part of the juridiction of the Roman province called Galatia then, it was isolated from the Galatian heartland by the desolated centre of the Anatolian plateau (lake Tatta, present Tuz Golu):
Strabo, Geography, 12, 5, 4: "After Galatia towards the south are situated Lake Tatta ..."
Furthermore, the area to the south was mainly populated by descendants of two ancient nations, the Lycaonians in the flat & arid East and the Pisidians in the mountainous West. These appellations show in 'Acts' for the first journey (13:14, 14:6,11,24) but NEVER "Galatia(n)"!
Strabo, Geography, 12, 4, 10: "to the south of the Paphlagonians are the Galatae; and still to the south of these two is Greater Phrygia, as also Lycaonia, extending as far as the Cilician and the Pisidian Taurus."
It appears that these southern parts were not considered then as inhabited by 'Galatians'.
However in Ac16:6, when Paul & companions move away from this region towards Troas in a NW direction, then "Phrygia and the Galatian country [can be translated as "Gallic ('galatikos') area"]" (Darby) are mentioned (as east of the province of Asia and south of Mysia & Bithynia --Ac16:6-7).
F) How could Paul write "You foolish Galatians ('Galatai'=Gauls)!" (3:1) to city dwellers of "southern Galatia", NOT ethnically Galatians? More so when these "Asian" Celtic Galatians were still known as uncivilized rural barbarians.
E) 'Galatai' (nominative/vocative forms) is also used by Polybius (circa 150BCE) and Pausonias (circa 150CE) in these forms or others ('wv'(genitive)/'ais'(dative)/'as'(accusative)) to mean "Gauls", either Asian or European (from the West!). Here are some examples:
Polybius' histories (II, XVIII) "This gave the Romans time to recover their strength, and to come to terms with the people of Latium. When, thirty years after the capture of the city [Rome], the Celts came again as far as Alba, the Romans were taken by surprise; ... But when another invasion in great force took place twelve years later, they did get previous intelligence of it; and, having mustered their allies, sallied forth to meet them with great spirit, being eager to engage them and fight a decisive battle [348BCE]. But the Gauls ['galatai'] were dismayed at their approach; and, being besides weakened by internal feuds, retreated homewards ..."
Polybius' Histories (XXI, XXIII) "While the negotiations for peace with Antiochus, and for the settlement of Asia generally were going on at Rome, and the Aetolian war was being fought in Greece, it happened that another war in Asia, that, namely, against the Gauls ['galatas'], was brought to a conclusion, the account of which I am now about to give."
Pausanias' Description of Greece (I, IV) "These Gauls ['galatai'] inhabit the most remote portion of Europe, near a great sea that is not navigable to its extremities, and possesses ebb and flow and creatures quite unlike those of other seas [Atlantic ocean] ... So they tried to save Greece in the way described, but the Gauls ['galatai'], now south of the Gates, cared not at all to capture the other towns, but were very eager to sack Delphi and the treasures of the god. ... The greater number of the Gauls ['galatwn'] crossed over to Asia by ship and plundered its coasts. Some time after, the inhabitants of Pergamus, that was called of old Teuthrania, drove the Gauls ['galatas'] into it from the sea. Now this people occupied the country on the farther side of the river Sangarius capturing Ancyra, a city of the Phrygians, which Midas son of Gordius had founded in former time."

In conclusion, the first time "Luke" considered Paul going through part of Galatia, and alone, and having disciples there, and spending time then, is at the start of the third journey (Ac18:23, 19:1).
The place Paul had to stay was probably in central/western "northern" Galatia, as shown on this map:

Notes:
a) "Luke" suggested Paul had already disciples there (Ac18:23b); but it is rather unlikely because no preaching was reported earlier in that region.
b) Paul's itinerary through "northern" Galatia was only about ninety miles longer than the one through Iconium. Why would Paul make this detour is subject to speculations. Maybe it had to do with his recent dispute with Peter & his rejection of James' directives (Gal2:11-14), separating Paul from the "Nazarenes", Barnabas & the other leaders in the Church of Antioch. Therefore many Christians of "southern" Galatia (converted earlier by Paul & Barnabas, under the auspice of the Antiochene church) may have turned against Paul, causing him to avoid the area.
c) The expression "at the first" (Greek root "proteron") does not imply Paul preached again to the same people. 'Proteron' normal translation is either "before" or "prior", therefore alluding only to Paul having done the initial preaching to them.
More details about Paul's third missionary journey (and time slot within which Paul was stranded in Galatia) on this page.

C) There is nothing in 'Acts' or 'Galatians' to indicate the later was written soon after the conversion of these Galatians. "so quickly" (Gal1:6) is indefinite and its interpretation as meaning one year or less is arbitrary. Furthermore, if the time in question was short, it would have been advantageous for Paul to declare it, but he did not! Anyway, in verse 1:6, Paul is exasperated about those abandoning him for other preachers and would have said "so quickly" regardless of the elapsed time (one, five, even ten years).

D) Titus is mentioned in Gal2:1-3 as a companion of Paul during his trip to Jerusalem. However, Titus is not in 'Acts', even if other companions/helpers are named with Paul during his first journey (Barnabas) and the second one (Silas & Timothy). But Titus appears prominently in '2Corinthians' (written during Paul's third journey) as the main "helper" towards the Corinthians:
2Co8:23 "As for Titus, he is my partner and fellow worker among you;"
That would imply a late dating for 'Galatians'.

Note: the "council" of Jerusalem, as described in Gal2:1-10, occurred after Paul went to Europe (second journey). Therefore it is likely this (uncircumcised) Titus was one of Paul's converts from over there. More justifications on this page

E) Eligibility of Gentiles (and Jews) in the Christian church & for salvation, with calls for dumping the law of Moses, is the major theme that 'Galatians' & 'Romans' (written early 58C.E.) have in common and was probably the major issue then that Paul had to face, as corroborated in 'Acts':
Ac21:21 "... you [Paul, months after he wrote 'Romans'] teach all the Jews who are among the Gentiles to forsake Moses. ..."

F) Other similarities with 'Romans':
a) Mentions of 'Abraham':
Gal3:6 "just as Abraham "believed God, and it was accounted to him for righteousness.""
Gal3:8 "preached the gospel to Abraham beforehand, saying, "In you all the nations shall be blessed."" also 3:7,9,14,16,18, 4:27 (8)
Ro4:1,2,3 "For what does the Scripture say? "Abraham believed God, and it was accounted to him for righteousness.""
Ro4:16 "Abraham, who is the father of us [Christians] all (as it is written, "I have made you a father of many nations")"
also 4:9,12,13, 9:7, 11:1 (9)
The only other occurrence of 'Abraham' in all the Pauline letters is in 2Co11:22.
b) Christians or Christ as seed of Abraham:
Gal3:16,19,29 (3)
Ro4:13,16,18 (3)
This does not show in the other Pauline letters.
c) Mentions of 'Isaac':
Gal4:28 "Now we, brethren, as Isaac was, are children of promise."
Ro9:7-8 ""In Isaac your seed shall be called." ... but the children of the promise are counted as the seed."
'Isaac' does not appear in the other Paul's letters.
d) Mentions of 'sons of God':
Gal3:26 "For you are all sons of God through faith in Christ Jesus."
Ro8:14 "For as many as are led by the Spirit of God, these are sons of God."
also 8:19, 9:26 "... sons of the living God." (3)
In Paul's other letters, there is no mention of "sons of God".
e) Mentions of '(un)circumcised' and '(un)circumcision' (multiple occurrences in one verse not indicated):
Gal5:2,3 "And I testify again to every man who becomes circumcised that he is a debtor to keep the whole law"
Gal5:6 "For in Christ Jesus neither circumcision nor uncircumcision avails anything"
also 2:3,7,8,9,12, 5:11, 6:12,13,15 (12)
Ro2:25 "but if you are a breaker of the law, your circumcision has become uncircumcision."
Ro3:30 "there is one God who will justify the circumcised by faith and the uncircumcised through faith."
also 2:26,27,28,29, 3:1, 4:9,10,11,12, 15:8 (12)
In the two Corinthians letters, these words occur in only two verses:
1Co7:18-19 "Was anyone called while circumcised? Let him not become uncircumcised [let him stay Jew]. Was anyone called while uncircumcised? Let him not be circumcised. Circumcision is nothing and uncircumcision is nothing, but keeping the commandments of God [part of the law of Moses] is what matters."
Let's notice the progression from '1Corinthians' to 'Galatians' & 'Romans'!
f) Mentions of 'law':
Gal2:16,19,21, 3:2,5,10,11,12,13,17,18,19,21,23,24, 4:4,5,21, 5:3,4,14,18,23, 6:2,13 (25)
Ro2:12,13,14,15,17,18,20,23,25,26,27, 3:19,20,21,27,28,31, 4:13,14,15,16, 5:13,20, 6:14,15, 7:1,2,3,4,5,6,7,8,9,12,14,16,21,22,23,25, 8:2,3,4,7, 9:4,31,32, 10:4,5, 13:8,10 (53)
In 'Galatians' & 'Romans', the law of Moses is most often described as a curse or bringing death & sin, all of which rid by Christ.
Gal2:16b "... for by the works of the law no flesh shall be justified."
Gal2:21 "... for if righteousness comes through the law, then Christ died in vain."
Gal3:10a "For as many as are of the works of the law are under the curse ..."
Gal3:13a "Christ has redeemed us from the curse of the law ..."
Ro3:20 Darby "Wherefore by works of law no flesh shall be justified before him; for by law [is] knowledge of sin."
Ro7:8b-9a "For apart from the law sin was dead. I was alive once without the law, but when the commandment came, sin revived and I died."
Ro8:2 "For the law of the Spirit of life in Christ Jesus has made me free from the law of sin and death."
Ro10:4 "For Christ is the end of the law for righteousness to everyone who believes."
But in the two Corinthians letters, where 'law' occurs in seven verses only, the law of Moses is not described in unfavorable terms (except in 1Co15:56, most likely a later interpolation. See here for explanations). More, it is even quoted by Paul in order to serve his purpose, twice (1Co9:8-9 & 1Co14:21)!
1Co9:8-9 "Do I say these things as a mere man? Or does not the law say the same also? For it is written in the law of Moses, ..."
g) God sending his (pre-existent) Son as a human:
Gal4:4 YLT "... God sent forth His Son, come of a woman ..."
Ro8:3 YLT "... God, His own Son having sent in the likeness of sinful flesh ..."
Only in these two epistles God is said to have sent his Son.
h) The word 'Scripture(s)' appears only in Galatians' (3:8,22;4:30) and 'Romans' (1:2;4:3;9:17;10:11;11:2;15:4;16:26), and not in any other authentic epistles.