...they're both children. Like, you act like this is a whole thing Nanase orchestrated to trap Yuni in this relationship when I just don't think that's the case. What's most likely her thought process is that she didn't prioritize Yuni enough so she's trying to show her more love and be a better girlfriend, however, she doesn't realize that the relationship has passed a point of no return and that Yuni doesn't love her anymore. She thinks she can put the relationship back together by getting better and being a better girlfriend, but doesn't know it's too late for that.
And, to be fair, Yuni hasn't really said anything to go against that and with Nanase being as emotionally intelligent as she is (not very), it makes sense that she'd trust that Yuni still loves her when she picked her over Fuuko. Honestly, I doubt Yuni even chose Nanase for the reasons you listed. I think it was likely a decision motivated by guilt. She knew her relationship with Fuuko was wrong and likely felt a lot of guilt about that. So it obviously makes sense that she'd choose the option that doesn't make her feel bad about herself. And, to address this now, this is easy to notice as an outsider. In Nanase's shoes, there's two reasons she wouldn't notice it. First, this manga has shown us countless times that she is not super emotionally intelligent, but also, when someone chooses you over someone else, is the first thing you do to psychoanalyze why they did it and find out if they were actually lying? No! They said what you wanted to hear, so you're just going to take the W.
So, in short... they're both dumb kids who aren't emotionally developed enough to handle this situation properly. And, honestly, if they were, this manga would've ended in Volume 1.