This is fuckign bs. The series has gotten a lot worst since it started with the sequel and all the cringe worthy forced drama. Especially with the obsessive roommate. No you don't get to determine if someone has the right to move out or not. Tell them straight out you had made this plan to move in with your "friend" before even starting school. Hell just say she is your girlfriend and be done with it.
The apartment gonna be gone and Yamada will be disappointed and hurt either way. I'm starting to think Yamada might be better off just breaking up with Kase and moving on. Kase knows Yamada has low self esteem and still gonna do this. She will end up thinking Kase doesn't even want to live with her now.
Of course it gonna be months till find out what happens.
The fact Kase is willing to bet it at all on her being able move in with Yamada right there shows she isn't serious about moving in with Yamada or about Yamada at all.
She is literally prioritizing a girl she has known for less than a year and always had a rough relationship with over her girlfriend of years.