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.
Precisely what Dammon implied. The only way i could see her heart being fixed or, hell, upgraded is only if she's in Avernus as it's the right temperature.
Also I think the safest bet would be to upgrade it instead of removing it. She puts stuff already that 'cools' down her heart so I don't see why it can't be upgraded somehow with better infernal material in Avernus without it going haywire. Besides even if Zariel did some magic doo doo that permanently has her infernal engine melded with her body, there has to be some solution. It's a fantasy world. Maybe she did something, akin to what Cazador did with Astarion - 'marking' it in a way so nobody can mess up with it. As her way of saying 'she's mine'.
A fight or a talk with Zariel would have been so fun to have in the game since we already can talk with gods and lich queens and devils. I'd have loved to see Zariel, honestly.
