Responses
I think his reasoning behind acting out was, he thought this will force his father and grandfather to reveal the truth that it was Yeshin who was the illegitimate child, not Hangyeol, to protect their reputation. Basically he was sacrificing himself so he would get kicked out and his brother would finally be accepted. It's immature but he was only 19 and emotionally starved of affection (as you said) so he did a reckless thing and it backfired.
Yeshin might be dumb but the ml is just as trashy as any other character. I seriously don't understand why yeshin thought if he acted out, slept around and be "the bad guy" his brother might get what was originally his, but maybe that's what his character is, plain stupid and dumb, starved of any affection and emotional support (since the mother left him) hence he was clinging to his rapist (the guy who abducted him when he was running away) And the ml is just as trashy, why was bro spanking Yeshin the first day they met, and even after the abduction/running away thing. Bruh even that was rape??!