Yeah karlach really suffered from being such a late addition to the companions. That being said personally I think the reason they havent given a quick fix is because she has such little content and they want to put alot if work into a dlc that will bring alot more content to the table for her. I don't have anything to point to other than a feeling really but it would make sense to give her something in avernus and an avernus dlc would be the perfect spot for it. I'd wish they'd do something about the plot holes in act 3 for her.
