2) Yes, other dwarven civilizations can come to trade, as well as multiple humans and elves, and maybe even goblins and kobolds (obviously, those two won't come to trade). Any civ on the 'c'ivs list should be able to visit your fort (though they may not for whatever reason). I don't know how you'd check which ones can come, though, until they actually begin populating your 'c'ivs list. Doesn't the embark screen only show which races can reach your fort?
As for civ growth, I don't really understand. Your fortress can grow in many ways, and eventually become the Mountainhome. The other civs, as per my understanding, are in stasis so long as you are playing, bar a few exceptions like named dwarfs that migrate to your fortress (King or Queen, etc). Is that wrong?
3) Quietust: Do dwarfs go insane between abandon and reclaim? I thought all creatures show up on reclaim exactly as they were on abandon, except with new equipment (or is that only for adventure mode?): melancholy -> melancholy; berserk -> berserk; normal -> wandering around the same few tiles until death, doing little more than nothing, not even dying of thirst. "All creatures" being only those that choose to stick around.