So many great questions! No worries about your initial wariness of her; I totally get it. The more minor ones first, in order: Her love language is quality time; I can't really think of any kinds of play off the top of my head that are "absolute no" rather than just "extremely bad idea but she could be talked into it"; in the Nullverse she's majoring in music; she doesn't cut Cy off so much as she just grows as a person and her life becomes fuller and takes up more of her time, as opposed to when they first got together and she was very isolated and didn't have much going on besides school and him. They definitely remain friends :)

As for how Isabelle came into my brain, of course given how dark many central aspects of her character are I'm sure you can extrapolate that there are some personal subjects involved I'd rather not get deep into. I will say that I mostly made her out of a desire to make some sense of things, to explore fear, desire, love, guilt, agency...all these things that her history and her relationship to her own trauma make her a very useful vector for delving into. At first, though, she was a much more shallow character and honestly kind of a twit! She was more Draco Malfoy-esque, very bratty and snobby and domineering. Then I realized what being the child of Alec and Viola Underwood would actually be like, and suddenly this whole well opened up for me to draw from. And now here we are!