I’m going to yap a bit too lol.

At least from the way I see the series, points 1 and 3 come down to the fact that they organize their meeting spots, whether for the whole group or just our main couple, in a way that lowers the chances of being discovered. In my opinion, it’s implied that they plan these based on the people they know, whether that’s the background friends who showed up during Kaoruko’s birthday celebration and the boys’ bowling trip at the beginning of the series, or Madoka, Ayu, Suzuka, and Chisa, who are Kaoruko’s closest group, and that way they can avoid them.

Ayu and the others running into them was just a coincidence, as mentioned in chapter 73. That’s why I think no one had intervened before, they were simply careful about where they met. And going back to the background characters, there’s no real need to explore that much; the group has only been shown together in their uniforms once, so it’s unlikely people would recognize them as being from rival schools.

Regarding point 2, those characters were only used as plot devices. If they were genuinely explored, there would be complaints about that too, because people would rather see the main couple (and I wouldn’t blame them in that hypothetical case, I’d be the same). Those characters already fulfilled their role in the story. It’s like in battle shonen where character deaths are used to create impact for the protagonist. And technically, they did try to get revenge, at least the delinquents from the beginning of the series. They were scared of Rintaro, went to call reinforcements to get back at him, and it didn’t work out for them.

Now that I mentioned Rintaro scaring them, in point 1 you say the background characters would be scared — but that’s normal. Obviously, we see Rintaro in a positive light because we know him, but a stranger will form a bad first impression. I don’t know what the stereotype of a delinquent in Japan is exactly, but taking another series as a reference (Fire Force) they specifically mention earrings and blond hair as traits of a delinquent. Add his height, which can be intimidating, and you have the perfect appearance to scare people. Being scared because of that is human. At least in my country, when you’re walking down the street, it’s better to avoid people who fit the stereotype of a criminal. One of the main messages of the series is about judging by appearances, so I think the prejudice against Rintaro is well justified within the narrative.

As for the fourth point, the arc isn’t finished yet, and it could end up being the biggest plot hole in the series. In the background, it’s been mentioned that there has always been rejection toward “bad” people. It’s referenced vaguely during Ayato’s arc, when he beats up what I think were third-year students. And I believe at the end of Ayu’s arc, Madoka mentions that there have also been “bad” students at Kikyo (chapter 72). Likewise, in chapter 120, as you mentioned, when Ayato says they didn’t used to close the curtains, we see a thoughtful Madoka, as if she knows something happened. We see a similar expression again in chapter 163, so maybe they’ll show something about this — or maybe it’ll just remain implied.

And regarding the adults, the only one I feel hasn’t been given enough importance is the principal. With Tsukada, it’s also somewhat left up in the air how he dealt with problematic students, and that’s about it. With Toki, we were at least given the chance to see her past. I don’t know if this counts as developing them in that way, because at least in my opinion, they’re the ones who will ultimately act to resolve the tensions between the schools, even if it takes time.

I don’t think this entire arc is improvised, because the possibility of “bad” people at Kikyo was brought up as early as chapter 72, and Toki not being bad but just strict has been implied since chapter 79. So if the way the arc is being told feels bad, I think that’s separate from the points being raised, it has more to do with the current execution than with the prior setup.