As Naoi noticed, there's something clearly off about Mona's home life that's causing her to be so desperate to bring Naoi back. Hopefully their mom's boyfriend isn't abusive to her in some other way if she was telling the truth about him not being physically violent. Their mom lied to Naoi when she made everything out to be perfect, since her and the boyfriend constantly arguing isn't going to be magically fixed by having a baby. If anything, it would only add more pressure.
I'm starting to suspect that their mom's motive, and to a lesser extent Mona's, is to recreate their former home dynamic with Naoi shouldering all of the abuse for them. Mona asking Kurumi to stay during her talk with Naoi could also be seen as manipulative, since she did it assuming Kurumi would side with her to convince Naoi to move in with her and their mom again. Hopefully they won't react badly if they find out about Naoi's plan to move out with Kurumi after high school.
Also, it seems Naoi's dad abandoned her without ever signing the divorce papers, since Naoi assumed that's why Mona came initially, to collect them. That could put even more pressure on her mom's relationship with the boyfriend, since they can't get married if she's still legally married to Naoi's dad.
As for Kurumi, this could be good for her and help her start to figure out how to face her own mother eventually.
And finally, Kokoro was in the color spread too, so she might get more focus this volume. Hopefully it'll be in a positive way and not something like her mistakenly thinking Naoi got Kurumi to ditch class this chapter to hang out.