Elsewhere he (Ibn Aabideen) says: "I say, and I have seen it in Kitaabul Kharaaj by Imam Yousuf that if a Muslim slanders the Messenger Sallallahu 'Alayhi wa Sallam or belies him (kadhdhaba) or finds fault ('aaba) or degrades (tanaqqasahu) be it known that he has disbelieved in Allah Ta'aalah and his wife goes out of his Nikah.. (Baanat minhu imra-atahu)" (Radd al-Muhtar vol.3/p.291)
"It is clear from this that those who slander the Angels or any of the Prophets (Alayhimus Salaam), he is declared a Kafir. And this is that type of Kufr where he is left off if he does Tawba, else he is executed (killed)." (Radd al-Muhtar vol.3/p.292)
"It is said in Bazzaziyyah from Khulasa that a Rafidi if he slanders the Shaykhayn [Hadrat Abu Bakr and Hadrat Umar RaDiyallahu 'Anhuma] and damns them (swears at them) he is a Kafir. If he only holds Hadrat Ali (RaDiyallahu 'Anhu) superior to him he is a heretic (Mubtadiy)." (Radd al-Muhtar vol.3/p.293)
And Ibn Abideen goes on explaining and debating at length about this and other such statements whether the slanderer of Sahaba is Kafir or not. If he is decalared a Murtadd; is he executed or left off after Tawba; whether his Tawba is accepted or not and so forth. He also gives a reference to a book by Mullah Ali Qari who is reported to have opposed the position held as quoted from Khulasa above in it.
Ibn Aabideen says, that even Khariji's are not considered as Kaafirs: "It is mentioned in Fath al-Qadeer that the Khawarij who hold that it is permissible to kill Muslims and take away their belongings (loot their maal) and call as Kafirs to the Sahaba, the ruling regarding them among majority of the scholars (Jamhoor) and the people of Hadeeth is that they are considered as rebels (Hukm of Bughat)" (Radd al-Muhtar vol.3/p.293)
But yet Hadrat Ibn Abideen gives HIS verdict on page 294, vol.3: "Yes, there is no doubt in the kufr of he who caluminates (Qadhafa) Hadrat Aayisha RaDiyallahu Anha by accusing her of inchastity or refuses to believe that Hadrat Abu Bakr Siddiq was a Sahabi or believes in the deification (Uluuhiyyah) of Hadrat Ali RaDiyallahu Anhu or in that that Hadrat Jabrayeel Alayhis Salaam did a mistake in bringing the Wahy or the like, it is open KUFR obviously (Min al-Kufris SareeH) for it (such beliefs) opposes the Qur-aan." further he says: "And the Rafidi who slanders the Shaykhayn RaDiyallahu Anhuma but yet DOES NOT caluminate Hadrat Aayisha RaDiyallahu Anha by accusing her of inchastity (Qadhf) NOR disbelieves that Hadrat Siddiq RaDiyallahu Anhu was a Sahabi etc., then it is not kufr and nor is it without Tawba but it is Heresy and Innovation (Dalal, Bid-ah) and the rest of the matter shall be discussed in the first part on rebels, Insha Allah (Bab al-Bigha)"
Note here that Ibn Aabideen, is debating on the slander of Sahaba not the denial of the Khilafat of the Shaykhayn RaDiyallahu Anhuma.
He says on page 310/ vol.3 - Bab al-Baghi:"It is clear from Kitab al-Musaayirah by consensus on the Takfeer (Ruling that someone is Kafir) of he who opposes the Basics of Religion (Usuul al-Deen) and the Obligatory matters [Matters which are obligatory to be believed in ] (Darooriyyat). For example that the universe ('Aalam) is from eternity (Qadeem) [without a beginning] or in the refusal of the belief that people will be brought forth with their bodies on Judgement Day. (Hashr al-Ajsaad)"
But then, we also have fatwas from other luminaries many of which I quote on the authority of Imam Ahmed Rida. (These can be found in the tenth volume of Fataawa ar-RiDawiyyah:pages 516-527 / Vol.10)
It is said by Shaykh Barjandi in Sharah Niqaayah:"He who refuses to accept the Imamah of Hadrat Abu Bakr as-Siddiq RaDiyallahu 'Anhu is a Kafir. Some opine that he is a Mubtadiy (a heretic) and not a Kafir but the truth of the matter is (As-SaH) that he is a Kafir. And so also is the case of he who refuses to accept the Khilafah of Hadrat Umar RaDiyallahu 'Anhu. And this is the most correct ruling."
Fataawa Bazzaziyyah states:"It is Wajib to rule someone as a Kafir (Yajibu Ikfaarihim) who calls Hadrat Uthman, Ali, TalHa, Zubayr and Sayyidah Aayisha RaDiyallahu 'Anhum Ajama'een"
It is stated in Bahr ar-Raayiq (page.131) : "It is the most correct ruling (in this matter; As-SaH) that it is KUFR to refuse to accept/deny the Imamah of Hadrat Abu Bakr RaDiyallahu 'Anhu and so also is the case in the refusal (of the Khilafah) of Hadrat Umar RaDiyallahu 'Anhu"
It is stated in Majma' al-Anhur:"The RafiDi if he just holds Hadrat Ali RaDiyallahu 'Anhu superior to Hadrat Abu Bakr (FaDDala) RaDiyallahu 'Anhu, he is a heretic; and if he denies / refuses the Khilafah of Hadrat Siddiq RaDiyallahu 'Anhu, he is a Kafir"