As far as Cassie goes, I'm a big fan of forgiveness, so I'm intrigued at the thought of her having a redemption arc. But I agree that it can't be as simple as 'she had a tragic backstory.' Her actions are clearly deliberate, and more importantly she consistently doubles down whenever trouble arises. It's clear she knows these actions have consequences for others, she just doesn't care. A broken childhood or the like might explain why she is that way, but it does nothing to change the fact that she IS that way. That's what would need to change, which would be a major undertaking for a side character. The arc needs to be long enough and thorough enough to convey that Cassie really has become a different person. Given that the process has yet to begin, Artemis had better be a very long game.
More importantly, Cassandra's atonement has to be something that can happen without romancing her. It has to be something that Cassie does on her own, purely to stop being a terrible person. If the MC/player decides to help her, or even finds that transformation attractive, fair enough. But Cassie has to complete her arc even if the MC remains unmoved - or flat out rejects the new version! That's the only way to show that Cassie is sincere about her penance. Portraying her redemption as the all-conquering power of the MC's True Love(tm) would fall extremely flat, IMHO, and probably wind up cheapening his relationships with the other LIs to boot.