A.The rule about "fewer" and "less" refers to things that can be counted vs. things that can't be counted. It doesn't matter whether it is more than than ten or not. If I had ten apples and someone took two of them, I would say that fewer than half the apples were missing. I would use "fewer" because I can count the apples. If instead I had made an apple pie, and some took a big slab of my pie, say about a quarter of the pie, I would say that "less" than half the pie was missing.
The rule about one to ten refers to spelling out numbers or using the figures. Generally you spell out numbers one to ten, but you can use figures for higher numbers.