Which is also why steel watcher tells her that her engine can't be fixed and she should go to have it dismantled.
Mild nitpick here, the Steel Watcher thinks that she's another Steel Watcher (it calls her "unit" and asks why she's walking with civilians), so it tells her it can't be fixed purely from the technological standpoint based on its knowledge. Not because it knows that she's a person and it's impossible to take it out of her to be fixed.
I do agree however, that even if the Gondians/Ironhands/Dammon do come up with a fix, the part about actually doing anything with her literal heart would still be the tricky one. It's unknown what magic Zariel and/or Avernus' mechanics used to rip her heart out and replace it with a machine without Karlach dying (so quickly, in fact, that she didn't have time to realize what was happening even). Plus, by Karlach's own words, we know that she's the only one who's not a devil and survived the engine. The "lucky" one.
Honestly, if THAT was our undoing, our insurmountable obstacle in her quest, it would make much more sense. Machines can be fixed, heart replacement with such unusual complications is another thing entirely.
But it would also make return to Avernus framed as looking for someone who can help, or maybe the operation would HAVE to be performed in the Avernus' temperatures so that the engine going crazy wouldn't complicate it further. Hell, the whole ending then would still be open, but have much more hopeful undertones, I think. Halfway there, a fix in hand, just need to bully a local mechanic into helping before Zariel catches on.
(But also having the exact parameters of HOW Zariel did it be vague and unknown technically allows for an alternative to exist in the material world as well, if the plot wanted it.)
EDIT: Re: "Otherwise why wouldn't she pull it out before it explodes" - well of course she'll die if she just rips her "heart" out and that's it. Replacing it, however, theoretically could be another matter entirely. Some stasis spell to allow her to hold on in the gap between uninstallation and installation, or something like that. We figured out heart transplants with technology, why can't it be figured out with magic and artefacts (theoretically)?
This is exactly my thinking thank you for putting it into better words than I could.
