But one of each pair (Koguma/Harumi) is in much shallower denial than the other (Hino/Chiaka) and seems to be getting rather frustrated at their partner’s continuing failure to just go for the yuri gold.
I don’t think this is exactly right. In the boob story Harumi has been pretty explicit in (thinking about) a romantic relationship with Chiaki while in Hino-san no Baka, Hino is the one with thinking about Koguma as someone she likes romantically rather than just for physical things.
The reason I say this is because Chiaki up until pretty recently was fine with just a sort of fwb type relationship and has only recently started to become conscious of Harumi outside of their boob groping relationship. Koguma on the other hand is still deluding herself into thinking she only cares about Hino as a friend or a fellow classmate, and even goes so far as to rebuke Hino for wanting to walk around hand in hand “because we’re both girls” or whatever, despite pretty clearly having fallen head over heels for Hino.