1. How many dwarf migrants should be accepted before I seal off the passage to the outside world?
How many do you want?
2. How many z-levels can a "Basically unbreakable" dorf fall before taking damage?
Last time it was rigorously tested, I recall bones starting to break
reliably at 4 z levels. Unless it's into water, which gave an edge. That was in .40d, however.
2.5. How many layers of cavern can collapse on the above dorf before he takes damage?
You get hit by even ONE whole z layer of natural rock and you're in trouble.
3. If there is no way to get to your dorfs, will besieging enemies mine into your fortress?
Nope. But as with many things in DF, the more accurate answer is
not yet.
4. I really don't understand how the perpetual motion water pump/water wheel works. Help.
Ehat's to understand?
Build a waterwheel. Attach it to a pump. The pump supplies the moving water to the waterwheel. The waterwheel provides like 4x the power a pump requires to operate. Just add water to jump start the process. Once started, it will never stop.
Best method of adding that initial water - a second screw pump, manually operated.
Alternatively, a bucket chain can work. Maybe.
5. Would a fortress constructed entirely out of wood float on water without being attached to it?
No. (Though again, the more accurate answer is
not yet.)
Wooden walls are just like any other walls. They don't even
burn, for Armok's sake.
6. Will a dorf with sufficient toughness and swimming be able to survive a dive into magma?
Well, a dive may be pushing it, and good covering of Steel helps, but magma is not
instantly fatal, just nearly so. At least last time it was rigorously checked. The physics of magma may have changed with the new release, and the introduction of the magma sea and whatnot. There have been forts with shallow magma flows guarding the entrance (so long as they have shoes, and walk fast, and aren't distracted...), though it never seems to end well.