I feel like she should probably have extra possibilities here.
'Fix my hand!' Should be on the list. Let Selune deal with that mark, or Shar remove her own mark. If Selune deals with it, maybe Aylin and Isobel show up in that cutscene to help fix it.
Divine Intervention would also be good enough to fix Karlach if she hasn't used it yet by the time the credits roll. The Infernal engine may be gone, but I bet direct Intervention from a deity will let her have a new heart.
Dark Justiciar Shadowheart might have extra options of another kind, or even a chance to use it twice if she then kills her parents off. At that point she's Shar's Chosen and should have the extra oomph to prove it.
