I don't think the majority of the R18 is canon. Ch 14 is maybe canon because it lines up with mc's suicide attempt and is titled "Flashback." That, and it's one of the less sexual chapters, even with the pantyshot and cleavage, though with how long it's been without even a mention, it's seeming less and less likely that it's canon.And I'll remind everyone that he had sex with her in every other chapter of that R18 version. The kind of sex you have when you're trying to have children.
Clearly he wasn't that affected by the stripping episode if he's willing to have sex with her, repeatedly. I don't know about you guys, but if someone sexually assaults me, I am not having sex with them out of my own volition.
I don't know why this gets fixated on-- it's exactly one occurrence in an extended bullying campaign, that doesn't even get alluded to in this version, even when he's flashing back to the bullying. It's as if you mean to imply the bully might have been "redeemable" if not for that event, as if the rest of the bullying he endured was chopped liver.
Between the thumbtacks, spilled milk, writing mockery on his stuff, and sexual assault, I think the sexual assault is the easiest thing to point to when it comes to examples of how extreme femc's bullying was. Most of the bullying has been implied so far, with mc's revenge scheme and comments, and only Ch 14 of the R18 and Ch 26 being explicit to my memory. People just conceptualize one big incident rather than many incidents repeated over the course of years more readily.