lets see.
1. I think they only leave if they get really really far into negative relations with you.
2. If he owns a town or castle, he'll take it and any villages. If he only has a village, you get it back. This is why you only give walled center to people you know like you a lot, like if you have high honor, honorable lords are a good choice for walled centers because as you gain honor, they like you more to counter the rep hit if they lose points for any other reason.
3. Tough one. If you take a lot of land, you are gonna end up with guys running all over the place. There's no way to reallocate fiefs. Their army size is related to their income, which is obviously related to what fiefs you give them.
4. If you give walled centers to a lord, he gets more troops than if he just has a village. Also, he staffs it, which is nice if you aren't rolling in insane amounts of cash.
5. I don't think so.
6. More vassals = more quarrels, but also more armies. More fiefs per lord should = more troops per army. Which are also gonna be slower. That's a tough one.
7. I don't think it makes them more likely to defect, but it might save you some quarrels.
8. Sadly, no.
9. Yes, but as far as I know the improvements do stay active if you already made them.
10. They make their own according to their income (as above), based on fief ownership.
11. No. What matters is the size of his armies (which you can control in a sense), and whether he will defect (some are more likely to like you based on their personalities.
12. You lose them as a companion. You want them to be high level as level translates into their initial renown, and renown helps army size. Some non-noble companions may have you taking flak from other lords.