I dont know about the others, but for me it comes from a frustration of inequality. straight romances so rarely have side couples that are gay, even when they accidentally write a very queer-coded character. I feel like if characters were allowed to be gay outside of queer literature I'd have no issues here
that said, I don't think I'm dropping this, at least not yet. despite my complaints and frustrations, I am invested in things enough that I want to stay. this chapter likely just flagged more grief than the others because none of our previous characters showed themselves here
to your second paragraph - chapters 7-10 didn't have Kaede or Nami in them whatsoever, until the final to pages of chapter 10 when Kaede is contacted because Nozaki hadn't shown up to school for three days.
And that whole arc between Kuritani, Nozaki, and Kaizuka was set up at the end of chapter 5 with Katou telling Kaede about Kuritani being detained by police, with Kaede going and getting Kuritani, then talking with her about why she did what she did--seguing from there into Kuritani's story involving her troubles with love and how she just doesn't feel it, feels left out and excluded because of it, and then her falling into Nozaki's clutches over being spotted going into the hotel. Which lead into the Triangle situation between them becoming friends, until Kaizuka enters the scene and Kuritani realizes she's fallen for him.
And that basically took all of volume 2, with the "intro" to the Kuritani & Nozaki's story from the final bits of volume 1. And even then, we have both Kaede and Nami coming in, to assist both Kuritani and Nozaki (or at least attempt to) with the issue between them and the boy they both like.
And
this current arc begins at the end of volume 2, with a harder/more detached segue, but with a character we do have introduced before (Katou-sensei at the end of volume 1). But with volume 2 as precedent, there's no reason to believe that Kaede or Nami are going to be absent from this; it's just literally the first chapter of the third volume, setting up the current situation and conflict. Which, itself, isn't new to the story--the series begins with Kaede herself being confessed to by a student, and the teacher she goes to for help has himself been confessed to countless times--and as I said in my initial comment, we see what happens when "feelings are reciprocated", by way of the news broadcast in chapter 2 playing the background where teachers are arrested for having affairs with their students.
And it
is a student confessing to a teacher here. But unlike before, she actually reciprocates--but for reasons
other than returning his affection. I'm of the opinion that Katou-sensei is jealous of Takaoka's talents, and the fact that even at his age, she can see that he has something she feels she didn't. There are even other students whispering about how much better he is than their own teacher, and her partner also very much minimizes her abilites.
That all said - this has been foreshadowed to a degree as far back as the beginning chapters, and we have a pattern wherein the "main couple characters"--Kaede and Nami--do factor into the stories of others of the cast. But the scenarios have to be established, which is what this chapter does. From there, the drama can unfold, and as they are undoubtedly discovered, Kaede will absolutely get involved (given she seems to be someone intent on involving herself in others' problems, as she's a "helper", and because she herself has some experience with what Katou-sensei is going through, as far as the student-confession aspect).
Like I said - I can understand the trepidation, and your comments about how there's no real representation in "the other direction" that makes a primarily queer author putting het couples in a queer-centric story "agreeable"...but it wouldn't be the first time I've seen authors known for het work doing queer stuff, or vice versa, so putting both into the same story that works off an ensemble cast makes sense to me personally, I guess--or at least doesn't feel wholly out of left field.
I just very much hope that it doesn't get dropped by Cuetie Scans, because I don't know who else would pick up a story like this or that it'll get an official English translation, and I'm very much enjoying it.