You don't have to apologise every time you offend someone. Some people are easily offended and don't deserve apologies, and sometimes it's just not your fault at all. Also, the "otherwise you won't be popular" argument only applies if it's to that girl. Otherwise it's just a general statement about treating people with respect, not girls in particular.
While she's reasonable, she's not always so tactful about pushing their "destiny" on him. If you push too hard, the response can often be to push back, rather than to follow along.
"Gunner" is fine. I don't think there are any important lost connotations. Also, seems like an edgelord, but you never know in this manga.
Shit is starting to go down? Also Yuuto is relatable af, being aware of your own breath during awkward silence oof
Is this something I'm too introverted to understand?