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.
I agree with your failstate label for Avernus, certainly not a good ending. I believe there should be a good ending state for Karlach as an option. I do also think however that Avernus is the elephant in the room, it is after all the root cause of the problem. I don't think any solution be it past, present or future can be avoided, it has to come into play.
A fix for the infernal engine, peaceful living in Faerun, adjusting to the new lifestyle out of the hells seems to be what people would like to see as a good ending. It would certainly be viable but also leave the unfinished business as open-ended content, let the headcanon theorycraft. But the inevitable is Karlach is/was/will likely be an object of interest to Zariel/Avernus in some form, be it her own loose ends renting space in her head or it coming back to haunt her in some fashion.
There is potential for DLC or definitive content to make its way in and I believe that will likely take the form of some aspect of Avernus.
