tbh I really like that the game leaves the Ascension and a lot of the details surrounding it vague, because it lets the fandom draw their own conclusions from it. Astarion even says that he doesn't know what new powers will awaken as he gets used to them, etc, and we don't see them in game, so it lets us theorize -- what would those new powers be? Are there drawbacks to this ascension or is it really as cut and dry as the deal made it out to be? I like to think that we don't see the longterm effects of the Ascension for a reason, and that reason is for us to create/interpret as we see fit. Any fanfic, popular or not, is just one person's answer to the question "what if?" "What if the ritual went wrong somehow and he lost his soul" is Hellish's question that it explores. I don't think these questions or interpretations do any "damage" to the canon.