-all of my bins are wood. (so they will presumably scatter their contents across the map)
Probably.
-all of my buildable artifacts are built (a few are crafts, and in the bins) will built furniture (depots, bridges, screw pumps etc) stay built?
Everything will stay built. Furniture, buildings, constructions. Well, unless in certain situations where they would have deconstructed anyway, like hanging machines losing their designated, but not yet constructed supports.
Artifacts may be bugged, though. They'll still be artifact level, with the usual restrictions, but they won't appear in the 'l' screen. At least, mine don't (40d). I don't know if their value is preserved, but why wouldn't it be?
-I have 50-some dwarves and a dungeon master, where do they go? When I reclaim and raise the population again will I get the -same- dungeonmaster or another randomly-generated one?
They may stay, or they may disappear, and they
may go crazy. Any creature (dwarfs included) that stays on the map after reclaim is no longer under your control and will not count as part of your population. In fact, most have no control whatsoever, and will just wander around a small space doing absolutely nothing (see wiki for some exceptions). Any creature that doesn't stay is history.
Most nobles are random, aren't they? You'll always get a fresh one. Am I right that only the King or Queen are one-offs?
-will magma pipe creatures respawn?
Vermin like fire snakes? Yes. Creature like fire imps, magma men, etc? I don't know.