Eh, it could be a moderately interesting conflict. Sade is and always has been fucked up, way before she even met the Duke's grandfather. Her introduction to him is already pretty psychotic, and Daleth is already disfigured, so even then she was past redemption.
But on the other hand, after 170 chapters of searching and not finding a way to break the Duke's curse, the only solution does seem to be to get her to a state where she never even cursed him in the first place. Which means you either have to kill her before then (even disregarding the fact that the characters are too nice to time-murder someone, she's at the peak of her power probably, so not realistic), or find a way to convince her otherwise, which would likely be either a flavor of redemption, or at least something to resolve her jealousy.
My guess at this point is that our heroes will empathize with her in the past, she'll realize that she's a huge psycho bitch, and through time-fuckery will render all her misdeeds undone by just deleting herself from the timeline before everything else happened. So no disfigured Daleth, no curse, etc. That way all problems are solved and she's "redeemed" but also not getting a happy ending.