Honestly, after experiencing some of the other companions’ endings, I feel like the creators of the game intentionally make it so that there isn’t a happy ending for anyone. There is always some kind of painful trade-off, deeply imperfect resolutions. Sacrifices that have to be made. And since they said that the Karlach death ending was planned from the start, they’re probably still taking that approach to her as well. It seems like they want her to have the painful trade-off between being alive and being in Avernus, etc. Sad to say. This may all be as intended. That still leaves her underdeveloped quest-line. Not sure whether that was a part of the creative vision.