Reading through the chapter again, I think the point that Sayori is trying to get across more is the importance of communication. Perhaps we'll see how Mizuki and her were able to actually work through things, how this moment was a sudden loss of composure for Mizuki, and how they worked it out. Maybe they didn't, though that brings into question the whole double date.
What I think is more important is that, despite all the secrets and the obvious intimacy between the two of them, there seems to be a mismatch in what the relationship means for them. We have Sayori's perspective: it was something special, something secret, something no one else had, and that put her above others. It gave her a sense of specialness or even superiority that let her feel like she wasn't just another face in a crowd. She obviously also likes (liked?) Mizuki romantically, but we also see that as she's considering her future she seems quite okay with letting go of this if it means she gets a new special feeling. The college Mizuki gets into is apparently nothing special, and as much as Sayori seems to want to appease her parents, she also openly states she hates the idea of going to some random, no name college, even if Mizuki is there and waiting for her. Sayori desperately wants to feel special, even if she struggles to take the steps for herself, and this overrides her feelings for Mizuki in a way that seems like she doesn't even really consider what Mizuki's feelings are.
And what do we really know about Mizuki's feelings? She obviously felt very strongly about the two of them's relationship. She went to a college she doesn't seem to enjoy, began renting a two-bedroom apartment by herself, all for a degree she's been shown to not be particularly skilled in, all for the sake of being able to be together with Sayori. Sayori brings up the fact that people talk about Mizuki behind her back, make fun of her, call her names. We don't know if she's outright bullied, but it certainly seems like her relationship with Sayori is the one thing in her life that brings her unmitigated joy. She probably enjoys making art, even if she isn't very good at it, but that brings dismissal and ridicule. Sayori accepts her, loves her, and brings her joy that no one else can take away.
Except Sayori can rip it away from her. To have the sole source of happiness, of joy, of acceptance in your life suddenly come to you and try and rip away that warmth, all so they can go somewhere else to feel special about getting into a "good college", after everything you've done? Everything you've sacrificed and committed to get to this point? Sayori herself said she liked the idea, even if we know that Sayori didn't really think it was realistic. The two of them have utterly failed to communicate just what this relationship means to them. Sayori oversold her investment in their relationship, and I can't exactly blame Mizuki for blowing up like this. I don't condone the methods, but I can understand the reasoning.
I think the important thing is that Kasumi needs to learn how to convey what she's feeling. She needs to open up more about her insecurities and seek reassurance from Haruyo. But, that's a difficult thing to do. Her friend's suicide is still hanging large, and we still know very few details about that, or about their relationship.
I do like how these things aren't entirely cleanly resolved. Haruyo doesn't just swoop in, get Kasumi to suddenly open up, and then solve all her worries. There's a realism to Kasumi's self-loathing and Haruyo's inability to dig into it and resolve it. When Kasumi starts being distant, Haruyo simply tries to brute force closing the gap with physicality, because she doesn't really know what else to do. She's only a middle schooler, for God's sake. There's an emotional immaturity to all the characters involved, and it feels very real, and on rereading Vol2, I think I really like it.
I'll certainly be much more aware, I didn't catch much of the darker implications in earlier chapters (aside from the suicide but that's not really an implication), but like others have said I think, or at least hope, this will be a story about breaking a cycle of tarnished relationships.
