If all of Daggerfall's endings are true due to the warp in the west, though, doesn't that make it so that the versions where the player character didn't die are also true? When one possible outcome of something involves a person dying, and then all possible outcomes happen (even those which by nature contradict each other.) what happens then? Or did Bethesda avoid such potentially headache inducing lines of thought by just saying that if the character dies in one version of the ending it equals dying in all?
In any case, as you said, it's so the Nerevarine can be taken out of the equation so Bethesda doesn't have to address the of meeting the Nerevarine (or maybe a setup for a game set in Akavir, but most likely the former.) as to a lore reason for the Nerevarine's actions, that's probably up to the player to decide, after all, the Nerevarine is whoever the player decides the Nerevarine is, so naturally, what would motivate the Nerevarine's actions, and what Nerevarine does after reaching Akavir or what happens to him are all up to the player to decide.
The warp in the west had every changet that happened happened, and the constants were overpowered byt the change. one such change would be death in reference to life.
possibility
The Nerevarine clearly has no sense of priority.
He left at a time when Morrowind was in political turmoil, when they needed the Nerevarine (and Hortator) to keep things politically stable. So he abandoned them - and soon after the Nords attacked, and then the Oblivion gates started opening all over Morrowind.
Two words: Epic fail.
Yes, he left at the worst possible time, but he didn't know it. he left before the oblivion crisis, and the Redorans and Indorils (the 2nd and 3rd most powerful houses) should be able to fight off king Horogther, especially with the help of Dres.