The problem I have with this concept is that any ending where Karlach is forced by circumstance to return to Avernus will still feel to me like a failstate. It still leaves her main character arc inevitably out of player hands because the only difference is how she feels rather than being presented with an actual choice. Avernus is still the fountainhead of her trauma, and a satisfying story involving trauma should end with the character moving on, not accepting that this is the best they're going to get and going back.
I'm frankly not interested in being convinced that Avernus isn't that bad actually because at the end of the day, it's well... hell. There is a limited amount of good that can be done there, you can't make it a better place because it is, intrinsically, a bad place. In addition, Avernus isn't even the goal in this ending since we're merely there to buy time, leaving her character story STILL unresolved and out of player hands. If this is an RPG, I expect my choices to have a bigger impact than "this abuse victim still has to return to their abuser, but at least they're not sad about it".
A major grievance I have with this ending is the lack of player impact. Impact that, in my view, can only be gained by resolving her problem fully. Otherwise we may as well just leave her by the river where we first met.
