The only way this wouldn't seem plausible is if you were one of the Nene haters who assumed she was using Kou - even though it was obvious she had real emotion toward Kou as well. You were wrong and you need to accept it. Certainly an unexpected outcome, but only because we so rarely actually see it happen, not because it wasn't built up to. It does make sense, the groundwork for it WAS laid, and as such I'm totally happy with it. (Excepting the fact that the teacher didn't get a girl of her own, but.)
Thanks for the condescending lecture—your advice about what other readers “need to do” is truly patronizing.
There’s no doubt that the narrative possibility of a polyamorous outcome was established relatively early on. You may well be satisfied with that outcome. But it’s not the outcome itself that’s implausible; it’s the herky-jerky path it took to end up there.
The story as it was actually told was little more than farcical, with both the senpai and Nene first depicted as deeply disturbed (the senpai's desire to see her loved one cry; Nene’s “hatred” for Amane) and then everything reversed on a dime because perky little Kou won the photo contest.
Nothing like setting up an intense emotional dilemma, then handwaving it away with a bunch of glitter and floaty hearts.