Goddammit system stop flagging my post as spam. I know I write a lot.
While it's true the teacher admitted it was love at first sight when she first laid eyes on Emi, Keiko was the one who was SUPER surprised that Emi confessed to her. Keiko had to ease into her new role as Emi's love interest/date to respond accordingly. She explained/exposition to the readers it's just the way she handles romances. If she was asked out, she would say yes unless it feels off. The relationship would gradually develop over time. It never occurred to her that another woman would confess to her romantically. Love never entered into her brain. Whatever she saw as sparkling may just be a passing fancy, admiration for beauty, whatever it is. I can be making random assumptions and can't back any of them up.
The age insecurity I feel is very much justified. I think this may just be a very much cultural thing of the view/stigma of older women in their 30s. (Which I don't want to go into since I already write in paragraphs and I don't have the strongest understanding of Japanese society interpretation etc) Keiko is dating someone 10 years younger, so she may feel a bit insecure about her own body/image of how she may appear to her lover. She's dated people in her age group or older if we're going by assumptions, therefore not used to handle whatever younger partner's worries may have. So her insecurities and her mini fight with Emi feels pretty real. One's a working adult, the other's still a student trying to get her homework done in a timely manner. On your argument about the teacher's fluctuating emotional age of 17 to 40, love makes people do stupid/irrational things. Keiko was floundering around like a lovestruck idiot on new year's day when she was itching to bone Emi. She approached Emi to make up for the fight and offered very mature responses to Emi's concern about her male friends. She's not just going to drop all her male friends like that, that's just not how it should go. Cute girlfriend or no. Not sure what I can say about Emi being like a Mary Sue, so... shrug? The lead up to the conflict and the solution is believable, although it was resolved a bit too quickly and cleanly. But an adult should be capable of acting like Keiko so whatevs. If you really want to talk about flat characterization, you might as well look at Keiko's 2 housemates: "Love should make your heart race" or "love should make you wet." Although these 2 are more just exposition device rather than story mover.