What happens to their soul? We all know that when a Lesser Daedra dies, it's soul enters the Waters and it finds it way back out and is reincarnated. Can this happen because a Daedra either dies in a Daedric Realm, or because if there are open portals (Or a lack of Dragonfires), there's a path for the soul to enter the Waters.

But what if the Daedra is slain after Martin sealed the rift forever? Where there's no way into Oblivion except for arduous rituals and spellweaving? Would the soul enter straight to the waters or would it have to somehow escape Nirn first?

There are still paths between Mundus and Oblivion even with barrier in place; notice that the door to the Shivering Isles comes after the rift is sealed, and you can still summon daedra even after it is sealed. The only thing Martin did was stop another large-scale invasion akin to Dagon's.

As for how they depart, I'd argue one of two ways. Either they *poof* directly back to Oblivion like the reverse of a summoning, or (if Umaril is any model for regular daedra), then their souls have to literally travel the whole distance back through the sky. On that note, it would be very interesting to know if these souls have to deal with things like the Lunar Lattice, but that's unknown.

Going out is the same as it ever was. The barrier only restricts entry.

We also have Morrowind which might be a huge barrier-less area do to the large amounts of Daedric worship.

If they have to travel the whole distance, they have to get past the Aedric planets as well, which might well appear as the infinite planes they actually are to them. I'd go with poof.

I assume it's possible to get to Oblivion that way, else I have no idea what Umaril thought he was doing up in the sky above White-Gold.