AADA7A: Was it just a question of time, of accumuling enough clues of past incarnations, or did it actually take something from TNO maturity-wise? I mean was it through his companions, or finally meeting Ravel, or somesuch?
My guess would be - a combination of all that. The Practical Incarnation failed due to his approach to the problem, but the clues he left did prove useful for TNO. On the other hand, TNO would need to
want to follow these clues, to know and accept his past - the Paranoid Incarnation ended up the way he did because he just wanted to live his own life and couldn't stand being blamed for something he couldn't remember. Also, the fact that the "current" TNO no longer loses his memory upon death - something that may or may not matter in-game depending on whether or not you die at some point - is related to the
time that has passed since he was made immortal: Ravel's spell gradually got weaker and its effects changed.
AADA7A: And perhaps it is outside the scope of the game, but how come the different incarnations are so different from one-another? They have no upbringing so one cannot point to parenting and such. :P
This is a bit meta, yes. TNO's previous incarnations presumably awakened in the same manner he did - in an unfamiliar place, with amnesia. Since Torment is an RPG, having the main character start as a blank slate allows for different behavior and interactions. While you don't really have the choice of going insane, you
can be a complete jerk who puts his goals above everything else. So each of them chose their own path like that, sometimes starting with their reaction to their awakening in the Mortuary. (Morte even talks about how TNO often woke up with a different personality after dying.)
AADA7A: And also how many past incarnations have the companions which one actually employs within the game accompanied? Does it differ between say Morte and the others? I played a long time ago but I'm writing a text which should be submitted the 30th and I thought these details might be relevant.
We only know of Practical Incarnation's party, which consisted of Morte, Dak'kon, Deionarra and Xachariah. Morte and Dak'kon survived the Fortress of Regrets and were clearly waiting to join TNO again, and Morte has seen many of his "incarnations" already. If TNO had any other companions at any point between the Fortress of Regrets incident and the beginning of the game, they weren't important enough to warrant a mention, apparently.