1. How many dwarf migrants should be accepted before I seal off the passage to the outside world?
The question is not how many, but
which. If you are on a map with a volcano and sand everywhere, you do NOT want to exterminate that High Master Glassworker just because she's an immigrant. Same with HM weaponsmiths/armorsmiths.
Legendary cheesemakers are a different story.
And while YMMV, sealing off the outside world is not necessarily the best thing to do. You miss out on so much... Fun.
2. How many z-levels can a "Basically unbreakable" dorf fall before taking damage?
There is no such thing as an unbreakable dorf. If there was a dorf that could survive a 100 z-level plunge, he would probably die by tripping over a kitten corpse.
2.5. How many layers of cavern can collapse on the above dorf before he takes damage?
Generally speaking? None, IIRC. Some may heal fast with no lasting effects, but generally speaking, dwarves are not immune to the Rocks Fall, Everyone Dies effect.
3. If there is no way to get to your dorfs, will besieging enemies mine into your fortress?
In this version? No, not unless I've missed a lot. In fact, enemies without the ability to destroy buildings tend to be thwarted by a door that you tag "forbidden".
4. I really don't understand how the perpetual motion water pump/water wheel works. Help.
You got me there. I don't know how to make one, either.
5. Would a fortress constructed entirely out of wood float on water without being attached to it?
No, alas.
6. Will a dorf with sufficient toughness and swimming be able to survive a dive into magma?
oh god no and how.