I don't think you'd be retconning Karlach's entire arc, really. To me it feels more like actually finishing it in one way or another. The biggest problem I have is that her arc in the story just kind of stops in act 2. There's some great content for Karlach in act 3, but everything that actually has to do with her ending (engine failing, can't be repaired) has happened by early-mid act 2 and there's never any follow up on it. That's a huge part of what makes it so unsatisfying. It's like having Astarion inevitably being kidnapped and used for Cazador's ritual because there just isn't a quest to deal with the problem in act 3, with no reason why there isn't ever given.
