There really isny any plausible reason we *cant* fix her. Is the issue imo.
As so many ppl have said our characters are able to do some wild shit and put our companion characters on drastically different paths and for the most all of those paths feel satisfying. Depending on how you wanna do your playthrough.
Karlach gets none of that.
She really doesnt need a more justified reason for why she is boned.
She needs an option that the player can save her. Make it hard sure. If there needs* to be a trade off sure i guess. (Im kind of against that route cause ksrlachs personality seems like depending on that trade off she would be upset with it and not go for it) - but at the end of the day. She just needs a happy ending option. A satisfying conclusion. I will be pretty choked if they just add in dialogue or another quest log only for it to still end with -we did everything we could for karlach-