I find all those answers unsatisfactory, but in particular the defeatist attitude that we'll never hear from him/her again has me miffed. Nerevar is immortal. You don't just retcon out immortals. They shouldn't have made him immortal to begin with if they were just going to throw the character into the waste bin.
I don't have any constructive theories to add or monkeytruths to propose. I just wanted to put my foot down and say "nope, i don't like it, i don't like it one bit!"
I don't like it either, but there is a difference between not liking something and refusing to accept it. The Nerevarine is gone. I don't know where, why, how or for how long and neither does anybody else, you can make up your own story there or just accept that you will never find a satisfactory answer.
Of course the alternative is creating a massive fan uproar and trying to convince Beth to make a cannon story for the Nerevarine, but the chances of you succeeding with that are beyond small.