I understand what your saying, I also have anxiety, and in concept, this would be a nightmare for me. But the thing is that reality is a lot more malicious than the universe we were presented with. I think context matters, the uke was happy someone paid attention to his symptoms and made an effort to accommodate him. We can't apply real-world logic to a story where we know everything about these characters. Therefore we know if they are dangerous or not, unlike real life. We know the seme would never hurt him because we're in his head. I knew the uke would be safe so I really didn't find it that unsettling.
Once he finds out this man wants to be with him romantically and has felt this way from the beginning he is relieved because he feels the same way. The reciprocity matters. Also, even the seme himself acknowledged this wasn't the most ethical way of pursuing him. I don't think romantic and or sexual intentions are inherently bad, as long as they aren't being weaponized against the love interest. Anyway sorry I rambled.
It would be difficult to tell people's real intentions, I would assume that they would want something in return for helping me. That's why it would be stressful if someone were to approach me whilst I was having a panic attack. I never really had someone to rely on during these hard times, so I guess I became suspicious of people. But I understand that generally, it could be a good experience to have a stranger be kind to you in a time of need.
It's precisely why I love this manga so much, I get to enjoy this fantasy of someone not only caring about my health but also taking direct action to help me feel safe in public spaces. I get to dream that humans can be this understanding and helpful, even if it was driven by him wanting to date the uke, it was still beautiful to see. I legit cried
Yes! After all it's a romantic story, I know irl this could be way more open to discussion (and I understand ofc if someone still feels uncomfortable by reading this manga like this person did) but I personally don't think Niwa is a bad o creepy guy for approaching a man he likes and taking genuine interest in his disorder in order to make him feel comfortable (also he was okay if they only ended up being friends after all), at least that's the take I had on this manga.

Ookay this is totally a personal thing but if i was in Shiina's position i would have been very creeped out by Niwa? I'm not diagnosed with a disorder but i'm an anxious person who tend to overthink. If someone i met for the first time was suddenly everywhere around me all the time AND told me that they did a half assed research about my symptomps after a very short interaction (which -- i'm really surprised that Niwa got it right, what a plot convenience) i would totally feel uncomfortable??? not to mention that Niwa did have an ulterior motive. Author should have built their relationship more, made them be friends first and then let Niwa finding out about Shiina's illness be more natural and less creepy / predator-y. Also, Niwa totally outed another person without his consent, and yeah i get that the person in question was a total asshole but that didn't justify Niwa's action. I think. Lmaoo what do i know.