As for the specifics of Karlach. As a DM I would have allowed a way to "fix" her. However it would not be simple. A wish spell, as some have suggested, is SCARY to use and should be an act of desperation. Various other heal spells would require either a VERY high level priest and/or VERY expensive spell components and sacrifices. As with resurrections I would have required a very difficult task to be completed in order for the casting to be done. (money would not be enough) I would further add a time limit in based on Karlach's needs to add more "drama" to the questing. This is all however outside the realm of the BG3 project, would make a nice added content item
Honestly? At this point, i'd go 100% with that. Make us sweat for it. Or at the very least, add some 'trade off'.
