Your only option is to engage with in-universe systems. There a lot of failings of the D&D alignments mixing with IRL morality. You can disagree only as much as your god belief allows you to which, as demonstrated, can vary wildly. This is assuming you want to disagree within the game. If you as an IRL player don't care then you really can only go with kill or ignore.
I think this is where I have a profound issue, because the game is approaching the idea of redemption from a very specific IRL cultural/religious perspective. But since I don't come from that particular religious tradition, I am having great difficulty accepting it.
The game is?
So far I have only seen players approach redemption that way and speculating the game will allow for it. That is when they don't think she will discover she was a Selûnite all along and switch belief system just like that. That's not a redemption arc, it's just a toggle to make the character good or evil based on the player's whim.
The only way I see Shadowheart redeemed is after years of (conversion) therapy...and at that point, is it really therapy or good people brainwashing her to be good?
