Ah thanks.
I was under the impression that everything in TES was set in stone and was simply being "played out" by the people in it. I like that the world is more chaotic and changeable than i thought it was.
I'm pretty sure I've read that what different people read different futures from the Elder Scrolls (The actual items in the lore, not the games.) because the future is not yet set in stone, and only after the event has happened will the Elder Scrolls all tell the same thing to everyone, or something to that effect. I wonder if it's a coincedence that this seems to parallel how in the games, things will play out somewhat differently for different players depending on the choices they make in the game, but in later games, at least the main story of the game generally becomes more or less set in stone (Daggerfall is an exception, due to the whole Dragoning Breaking thing going on.)
As to what would happen if the Nerevarine was killed, I suspect the player character might simply join the ranks of the failed incarnates, after all, they are all people who tried to fulfill the prophecies, and seemed like they might, but failed before they could. While one might interpret this as simply because they were not actually the Nerevarine, but just appeared to fulfill some of the aspects of the pophecy (After all, if your prophecy includes things like "being born on a certain day to uncertain parents, there will likely be many people who fulfill that part while not being able to get the other parts.) and failed because they were not actually Nerevar reincarnated, one might also think of this as being that you are not actually the Nerevarine until you have successfully fulfilled all the aspects of the prophecy, in other words, you are, as one character says in the game, one who "Might become the Nerevarine", if things work this way, then if the Nerevarine fails, someone else who fits the signs could arise to take his place. If you're going to have a prophecy, it would seem to make a bit more sense if it leaves some margin for error like this, after all, having Morrowind potentially becoming doomed to the fate Dagoth Ur has planned for it because the player character missed a step, fell off a cliff and broke his neck seems like a bit of an impractical set up, though judging from a lot of prophecies in fantasy, one gets the impression that practicality is not high on the list of the priorities of the gods or destiny.
Of course, that's all guessing on my part, prophecies can be confusing things if you think about them too hard.