-Will enemies path through buildings, or merely destroy them if they get close?
Building destroyers will path TO buildings, others wont.
-Will enemies path through water or magma, and does it depend on whether or not they can survive in such?
Those that can survive will path, those that can't wont/
-If no dwarven targets are available, will an enemy seek out pets or animals to attack?
I don't believe there are preferences, they will simply attack the first target, dwarf, animal, merchant or building (if building destroyer).
-If no dwarven or pet targets are available, will an enemy intentionally seek out buildings or constructions to destroy?
See previous answers.
-Can moving water push such enemies?
Yes, in some situations (depends how the water is moving).
-Can falling water force a flying enemy down?
No idea. I'd guess at yes though.
-If so, can it do so with enough force to hurt them?
I've never seen any damage from water or falling through it besides drowning, so I'd guess at no.
-If a flying enemy is grappled and finds itself over open air (for instance, if a bridge is removed,) will it hold up its opponent, drop it, or will they both plummet?
No idea, this calls for !!SCIENCE!!
-Do "hot" enemies melt ice they come near?
If sufficiently hot, yes.
-Will enemies encased in obsidian leave a body that you can mine out?
No.
-Will enemies encased in ice leave a body that you can mine/melt out?
No.
-Can certain enemies "Phase" through fortifications? Without water pressure?
Yes. I am not going to elaborate.