I realize you're too busy for writing stories now, but if you ever pick up the Evil!Rosa story, I thought of a way you could do it with the supernatural that would make sense.

We know about Cecil and how he uses a dark power to fuel his Dark Knight skills, right? What if that same force saw how good Rosa was and knew it needed to corrupt her if it wanted to keep its hold in the world? With something that powerful, it would make sense for it to be stronger than Rosa's own purity and goodness and take her over. It may not be as dramatic as something like Golbez meticulously brainwashing her, but it's an idea!