Larian Studios

I mean, there's literally a thread that's over a hundred pages long that goes over, in great detail, about how there are dozens of possible solutions to the engine problem that the game just doesn't let you try. There's a reason folks are frustrated with the way Karlach's story ends.
Also, reading my 5e PHB, you wouldn't even need true resurrection, a basic resurrection spell would work (according to the PHB it "closes all mortal wounds and restores any missing body parts" which would presumably include Karlach's missing heart if you removed the engine from her corpse), and that's a 7th level spell. If we can't find at least one level 13 cleric willing to help us after we literally saved Baldur's Gate, there's something strange going on. For that matter, Shadowheart and/or the PC would quite possibly *be* a level 13 cleric not long after the events of the game.
Here's hoping we get some resolution to the problem in a later patch/Definitive edition.