I wonder if Larian had to "rush" the game out and hadn't quite finished all of the Karlach story paths?? Maybe that's why you walk past so many options that technically should be exactly what she needs to fix her heart, but the game simply ignores them. You have no dialog options with anyone to continue her quest.
If this isn't a case of Larian not quite finishing a character storyline, then it looks like they were wiling to create gaping plot holes in order to deliver a tragic story. Not goo writing. Plus, it's quite emotionally manipulative to be sure to take the sunniest, most enjoyable character on the team and off them, just becasue...
Based on some of the news, and some things Karlach's (stellar, wonderful) voice actress mentioned, it seemed that a lot of work on her came in the final year/months of development. So that is certainly a possibility.
