In this case there is pretty solid proof that the mom just intends to leave Kurumi alone indefinitely. Though, on second thought, Kurumi is probably old enough that this could be seen as justified. It's not like she's 12. There's also no school in session which means relatively less oversight.
I still feel like she's getting to the point where she can't make things much worse. Police don't normally help, but she's in a situation where more eyes on her is a good thing (barring a scenario where Kokoro is successfully cowed). She could also call police if mom self injures and there would be evidence in the injuries. The police will likely be called on her if she runs away anyway, so taking the initiative means she would get her story out first. Like pretty much everyone around her is insane and relentlessly drawing attention to that would give her slightly more leverage, as otherwise people are going to do terrible things to her anyway.
I understand why she doesn't call the police though. I wouldn't either. Feeling scared and disempowered, I would be inclined to try being on the run at least through summer break. Or, if it was me when I was Kurumi's age, to emotionally shut down and let Kokoro do what she wants to get the bad stuff over with.