The ONLY way I will accept a redemption arc for her is if she enters a nunnery and then we get some sexy times with Sister Mary Cassandra...I don't see an issue with Cassandra getting redemption or at least humanizing her and accepting she is a piece of shit. If she does get a redemption the best way to handle it would be to give a clear choice by the player to do so or not since it is such a crucial and sensitive area of the core narrative and the MC's development.
Oh and she becomes a ginger.
Even then I'd only do that arc after all of the others.