You just described why Vicec's fate is also left out of accounted events after Morrowind to allow further player freedom. Whatever the Nerevarine was and directly influenced is up to the imagination. Don't act like your version is any better than mine. Anything mentioning Vivec post Morrowind is only rumors and gossip. Thus the version where my Nerevarine threw the backstabbing fetcher into a Soulgem is just as plausible.
There's no such thing as "canon" in this series. Sure you have "history" that you can find in the books of the series. But anything directly describing the events of either game is nothing but a vague rumor left up to the imagination.
I was merely reinforcing your idea in that the Nerevarine's actions regarding Vivec is entirely up to the player. I offered my own Nerevarine's actions to support the fact that each player can have their own versions. You had yours, and I didn't mean to sound as though I was judging it. I was just saying that indeed we can each have our own theories and fates, and I presented my own as another example. I apologize if it seemed like I was attacking your own views.
And in response to your second statement; of course there is canon. But it as you say, anything directly describing the events of the games are vague rumors that we can build upon to fit around or with our own characters. My assumption that the Back Path is alternate to canon is solely based on it's name. It seems to indicate that it's another, darker alternative to the Main Quest. It just seems more likely that the front path would be the canon one. Of course, each person's Nerevarine could take either path, and that is why the events directly relating to the events of Morrowind are obscure. The canonicity here is left to our imagination.
Canonicity is very much present in TES, there is just less of it based around the events of each game.