Karond wrote:1.) Why do demons/devils when banished have to wait for a 100 years to get back? Is that a rule Ao or Azmodeus is enforcing?
Ao, I would assume. It's never really specified where the enforcement comes from, but it can't come from Asmodeus, as he is subject to the same rules. This is a core D&D rule, though, and could simply be a natural law of the multiverse, rather than something someone (deity or otherwise) has decided.
Karond wrote:2.) Do demons/devils technically have a soul? For devils its mentioned that souls are stripped bare with pain until they're little more than husks, before becoming lemures. That seems to imply that each outsider has a soul, no?
D&D seems very vague on what the definition of "soul" is. Devils are, as you point out, originally made from souls (unless they are birthed like children of other devils, as with Belial and his daughter Fierna who together rule the 4th layer of Baator.
Karond wrote:3.) What happens to devils/demons that die in their home planes? I believe I read something that dying there is the only way for them to actually die, but I'm a bit uncertain since devils in the blood war always fight on certain home ground so they come back if they fall. At any rate, following 2.), what happens with their souls when they die? All the other souls go to specific deities, but those from devils/demons are permanently destroyed? If they can be brought back even after that, which I believe I read in a FR novel a few years ago, where are the "souls" staying while they're dead?
A lot of the Blood War happens on the planes between Baator and the Abyss on the Great Wheel: Gehenna (mildly lawful evil), Hades/Gray Waste (neutral evil - native plane of the yochlol, who are also known as daemons) and Tarterus/Carceri (mildly chaotic evil). So most of the fighting happens away from their native plane, in which case they get banished when defeated.
But if they are indeed defeated and killed in their native plane, they are entirely destroyed, yes.
Where their "souls" go though? Well, I don't know. It is possible that if they have a soul (again, the vague description of this in D&D isn't helping. Maybe only mortals truly have souls?) it is entirely destroyed upon their death. On the other hand, they could have a similar fate to that of dead gods: They float as dead husks of themselves in the astral plane for eternity.
Alternatively, as we know, demons and devils seek souls of mortals as if it was money. Maybe if a fiend is destroyed on its native plane by another fiend, for instance, the latter absorb the soul?
Again, I can't find any clear sources on this, so it's kind of speculation.
Ending up in the astral plane is a fair bet though. It seems like the general dumping place for anything in D&D.