@PersonaJXT
I won't dispute the rest, but as for "why doesn't she/mc say anything?" "Why isn't anyone noticing how weird she's acting", I'll just say from experience as someone who has survived sexual assault, it's difficult. Very, very difficult, to talk about it. Especially so I imagine in Japan where the whole "shame" aspect comes in a lot harsher.
My experience was much younger than this so that might have been part of it, but I straight up repressed the memories for many years, until random things triggered me. Even when triggered though, I instinctively buried down the feelings because I was unable to deal with them. I didn't want anyone to know, even if a part of me knew I was the victim and not to blame for any of it. It was embarrassing and stressful to even think about it, let alone tell anyone. MC probably knows this, somehow, and doesn't want to "out" her. Irrational, yes, but understandable to some degree.
I also became very good at making excuses for any odd behaviors when I did get triggered. I remember having a meltdown in the doctors office because she wanted to do a physical examination and I convinced my mom the only reason I acted like that was because I was upset she took me out of school to go to the doctor because my class was having a party or something. There was no party. I took being grounded for misbehaving over telling my mother about what had happened to me years prior. Being punished was a better result than outing myself as being abused in my mind, and let me tell you I was a good kid who didn't want my parents to be disappointed in me. I still to this day have never told my direct family members about it because I don't want them to blame themselves for what happened.
It took until I was 22 years old to talk about it frankly, and even still very few people who actually know me and not just as "gomi-chan on the internet" know about it.
It's kind of like asking why the battered spouse doesn't just get a divorce. Its one of those things that no matter what your feelings are as an outsider looking in, you'll never truly know how you'll react unless it happens to you.