Do I really have to time and time again mention a god damned dating sim, because it did better job at storytelling than an RPG? How exactly should the good ending at the end, if an ending epilogue should be made to actually BE GOOD? How it can we a well written story if we are still ignoring billions of easy and lore accurate solutions?
The potential items like enriched infernal iron for example have a logical path to coming up with a fix sure. You have working infernal engines in Faerun via the Steelwatch that do not require Avernus' conditions to operate properly.
However, how do you integrate that enriched infernal iron? Does Karlach undergo surgery? The Gondians need to figure this out, it isn't just some steel chassis you're hooking into, is Karlach alive during this process, can she be? How do you remove the thing that is operating as her heart? It seems more complex than the heat shield plating reinforcement upgrades Dammon installed.
To make the story good and address the obvious bread crumbs that lead people to feel as you do, "infernal iron, Gondians, etc. Why can't we use this stuff" Larian is left with two options, the context obviously varying wildly:
1. Remove items that suggest there is hints to a fix, establish more definitively that Steelwatch Gondian technology is incompatible and Mizora refuses to help Karlach after the Gondians suggest we get infernal mechanics to assist. Make it so Mizora forbids ANY infernal intervention whatsoever, keeping a magical eye on Karlach, stopping all attempts to do so, hoping Karlach concedes and returns to Avernus. Karlach needs to be told no, these potential solutions are not viable, no god is answering her pleas, Wither says with all his power "No"
2. Utilize the already established fan theories or ideas.
a. Enriched Infernal Iron, Gondians workshop (don't blow up factory, forced to deal with steel watch during Gortash encounter as a trade off?).
b. Work with Gortash to replace Karlach's engine? He has the infernal connections, portal to new area of Avernus for materials using House of Hope portal?
c. Make a pact with Mizora and Zariel where Karlach is allowed to return to Faerun part-time in between blood war campaigns with a new and improved 2.0 engine that gives her power upgrades, Infernal Fury 2.0 (soul coin buff), makes her burn blue flame and white when excited "date scene". This way, Zariel's unfinished business can be addressed, rather than just brute force killing off Zariel's forces or fighting fire with fire in this case, Karlach realizes after killing Gortash there was no real satisfaction, just the same pain about what she had already lost, revenge didn't heal it. So you have character growth and taking the high road, coming to an agreement and working together with her slavers from a position of equality and power?
d. Any wish spells or true resurrections would likely be void of satisfaction, seems too silly to capitalize on that with such 'conventional' means at hand (plethora of suggestions from players, including yourself BitterSchizo). Yes I know, it is Faerun, DnD, magic exist and miracles but let's avoid another Gale fail safe Mystra situation, you'd be stepping on his toes too much, that is more his arc.
e. Withers decides Karlach can be summoned for a mere pittance 'semi-permanently'. Divine intervention is up there with wish spells etc. But I think this approach would be unique and not without compromise.
f. Gondians work with Dammon and Mizora, upgrade and fix Karlach's heart but Tav and Karlach agree to a one year contract to fight in the blood war for Zariel, after which Karlach and Tav can do whatever they want without conflict or trickery from Zariel/Mizora.
