You fight her and she LOSES, she doesn’t die. She runs away, basically, and there’s probably a chest that appears. Same goes for Anduin. You’ll “defeat” him, and then the Jailer will probably leave his body. Then Anduin recovers, but PROBABLY keeps his white hair. That would be my guess.
But honestly? After everything Sylvanas has done? The only way to redeem her would be to like, heal what is broken about her soul, essentially making her a completely different person than “the Banshee Queen.” She has her free will, but her soul itself is incredibly damaged by what Arthas did. If that doesn’t happen, then she needs to just buy the damn farm. No more genocide, no more stealing world leaders, no more lies.