The MC's reasoning behind not helping the female lead is that since he's not the original jackass who bullied and tortured the female lead therefore he has no obligation to help her. Which is such pathetic reasoning because there are lots of situations where no one is at fault but someone has to be the bigger person and solve the issue but in this case the MC expects the physically tortured, mentally ill and depressed woman to just "get over it" just because he's said sorry? Like excuse WTF what kind of self righteous idiot are you. This story is literally analogous to watching a person drown and even though you have the ability to swim you ignore the person because you have no obligation to help them; that's logically correct but no sane person would actually support you

In particular it shows that she is intent on getting more than a tactful apology from the protagonist for the previous character's actions---be it revenge or a sincere apology---and as a salaryman he probably feels like it is easier to wait, possibly putting up with a little suffering, until she gives up or gets bored, rather than infuriate her further by running away from the situation. However as a reasonable individual he has no intent to cater to her little revenge farce, particularly when he is not actually the guilty party. The fact that things continue to get further out of hand is a) an expression of the fact that the female lead is a spoilt princess and b) a design of the author that is consistent with the widespread taste for obsessive regretful heroines. Hopefully we agree that the latter is a slightly implausible character set-up and that most of the lack of naturality follows from there.

That is the development of the original novel---a kind of mutual redemption flow---feels more natural than the excessive regret she is currently feeling: what she has done as of chapter 57 is rather petty but not really so unforgivable, especially given the perceived past history. Of course she has lost face for her house, but this should hardly be a severe point of condemnation from either the perspective of a romance or harem novel.

The duchess attended an academy in disguise, and was ab*sed and bullied by the four contestants. She has staged this competition as payback: she puts the four through trials to make them feel the shame and hurt she suffered at their hands in the academy.

The author goes further to show she still has nightmares about the past ab*se, and suffers from what seems to be PTSD. And she's going to marry one of her ab*sers? Why?

The duchess has an "advisor" who tries to unravel why this is happening, and tries to make sense of the facts as he sees it.

MC has a reason why he doesn't just leave. It isn't simple because he is "s*upid/simpleminded". It's literally social su*cide on a medieval world not to mention this is a competition in which NO ONE has EVER decide to just give up. MC literally gives 0 f about trying to "act as the old him".

Female lead suffering from her memories and MC not caring is a lie. Because for one, MC truly doesn't care for the original plot of the story moreso because he took the place of one of the ab*sers. He isn't the ab*ser. He KNOWS why she choose the four participants in this competition in specific, he knows what each day "punishment" means. He knows the plot and knows that it IS a tragedy he doesn't want to be a part of. Because that is what the original plot was, a tragedy. Very early on when the competition starts we learn that that there is a "reason" and "purpose" beyond just marriage. Female lead suffering is mostly her agonizing on MC's change because of her vengance plot for him AND it is ALL mental, MC isn't a mind reader and we KNOW he is TRYING to see from her point of view but fails slightly because he is thinking on her reacting the old him, the ab*ser, not the him now. So all his moves made her THINK different and act different towards him which he obviously doesn't expect just from the first interactions this happen.

On chapter 32 we get three lines that sum it perfectly. How he isn't the sinner, how he doesn't care about this competition, doesn't care for her attention, this is her revenge and he just doesn't want to be a part of this even if he understand. Why he will not try to participate, why he wouldn't try to help her or try to be kind to her.