1. Probably, if they don't belong to a civ: many do. I'm not sure you can attack non civ sites unless you're looking for an artifact rumored to be there.
2. Only civs are organized enough to send sieges.
3. Subterranean animal people are always hostile (in theory, they often do nothing while still marked as hostile). They're probably incapable of occupying a fortress, and definitely shouldn't be capable of sending raids.
4. Only looking for artifacts, and I don't know if that's possible.
5. I think so, but haven't paid attention.
6. No. The only reason they bring food/drink is because the squad is defined to carry it (which is the default setting).
7. Unlikely. Raid battles work differently from fortress ones, and is much more abstract.
11. It's unlikely any direct thieving skills are modeled.
12. If you decrease the population of the enemy they will have fewer bodies to send in sieges send from decimated sites, and it may decrease the site pop enough that they aren't eligible army staging points (which may increase the chance of the main site sending a larger siege). Apart from inciting a war with previously non hostile parties it probably doesn't factor in the target selection logic.
15. It seems each raider only carries off one thing, so it might be the same with animals.
16. Cave critters aren't livestock but wild (with the Kobolds taming some to make them livestock). I doubt vermin can be stolen, but that's a guess.
17. I believe I've seen it, but don't quite remember. It might be artifacts.
18. Civs are separate, so yes, at least currently.
20. No. My experience is that I typically can get one back, and in lucky cases two. One has to remain to take the role of the site leader (which DF reports happens), so in theory all but one, but that doesn't match what I've seen in practice.
22. I assume someone else can conquer it. It should be possible for an uprising to overthrow the new government, but I believe that functionality is currently disabled.
23. I assume you can request any intelligent citizens/residents (I don't use mercs). I believe I've recovered naturalized human bards sent as soldiers. It doesn't make sense to request animals. If ghosts can be requested it should be a bug, as should night creatures, as they're not members of your civ. I assume you can get vampires back, though.
25. Probably not in most cases, since the titles are tied to official functions (like the soldier you lose as local administrator on conquest).
27/28. You can definitely steal everything they have available for theft, in which case you have to wait for them to produce new junk (a season or so).
30. Squads don't leave until all members have been collected, but wait off map while Urist McSluggard eats/sleeps/prays/reads... Death ought to result in the removal of that member from the squad, and thus allow it to proceed.
31. Not hunger/thirst. If you were to send them away such that they're gone over the new year I'd expect them to drop dead on arrival if they're dying of old age, just like visitors do.
32. No. It would be messy if some left via (different) caverns and some above ground...
33. As far as I understand they can't enter via caverns (never seen it or heard of it). You can get them to leave via caverns if you make sure it's the shortest path, and you can make them enter the map at a specific point(s) by blocking off the edges everywhere else (typically using bridges).
34. It's unlikely any of that is modeled currently.
35. Probably not.