Does no one else think that, regardless of whether or not Mahiro has feelings for her, she's kinda trying to put her on the spot to make her realize how her thoughts come across? Like she's forcing her hand so that she either decides to leave Kaori or realize that her thoughts and refusal to talk about it with Kaori are quite frankly pretty stupid and selfish.
I wouldn't put it this strongly, but yeah. Yukawa-san isn't being stupid or particularly selfish – Memo 12 starts with her musing on the fact that she's never really been in this position before. She doesn't really know what she's doing in this increasingly serious relationship with another woman, and is frightened of hurting Mizuki-san or getting hurt herself. She has a small inferiority complex due to how inexperienced she feels at her age (and given the age gap between her and Mizuki-san), and as a result tends to discount her own worth when set next to her girlfriend.
Mild spoiler: we're about to go into a flashback and see the history of Mahiro and Yukawa-san's friendship, which in turn puts what Mahiro is doing here into context. Mahiro knows Kayoko pretty well, and both likes her and isn't afraid to give her a semi-playful shove when she needs it. It's maybe a little pushy to put it in terms of "or we could date… just joking… unless…", but she's only human. I think it's more sweet than manipulative. One of my favorite things about this very fluffy but calm story is that there are no bad people in here.
Aside from the adult life / workplace setting, what I like best about this story is that it follows the relationship past its earliest steps into the much more treacherous terrain of actually doing the work to build a strong connection between two people, and that while the feelings are big, the drama is pretty small. I love the way the story shows two people, neither of whom are very experienced with relationships or love, who didn't expect to be in the kind of relationship that they're in, just trying to work it out as they go along. It feels very real to me, and the way they consistently treat each other with respect and kindness makes me root for them.