That would have been a lot more satisfying than what we have so far. Seems like you put a lot of thought into this and I like your ideas!
The only thing I am not entirely sure about is if the current ending options get somewhat locked out with that questline.
since you essentially either give her a premature death or progress towards the happy end, unless you completely ignore the questline. The return to avernus end would be basically gone, if you don't want to skip on a chunk of gameplay.
One option I could see is to simply be unable to fix her if all of the Gondians die, but Dammon telling her that she should try finding a master smith in Avernus or something along those lines, now that she has some blueprints or samples of an upgraded version. That would even open the door to some future Avernus DLC, if Larian were so inclined.
As for Step 3:
For the NPC we could be calling, another one that comes to mind would be Mystic Carrion - he evidently knows how to perform surgery. It would come with a price tag, and I'm not entirely sure what that would look like. Could be gold, could be something else.
