They're basically one of those overly saccharine couples that always goes in for things like matching mugs. Except they're able to take it a lot further, because they're both girls in the same grade at the same school. It's a little bit odd and it would probably be healthier if they grew out of it, but I wouldn't call it outright creepy.
The whole "This is me monopolizing her." thing was probably more about getting the other girls to back off on criticizing her friend for doing something they both enjoyed. Owning it like that pretty effectively prevents them from bringing it back to the other girl, even if it's more of a mutual choice in reality.
Finally, the confession at the end struck me more as a realization of what was actually going on the entire time, rather than a true change in the dynamic. They were basically acting as a couple, but they never really acknowledged it to each other. Hiromi going out on a limb like that pretty much confirmed it, for both girls, but she wasn't actually sure that Yuka felt the same way, so she got scared. Yuka made sure to keep up their routine and actually verbalize their feelings.