Ok so...
I read some korean review of the novel and apparently by the second half of the novel the ml is drowning in regret and does quite a lot of grovelling. At least that's what I understood from the translation. They also said that his character was not THAT bad, but they didn't go into detail. Take it with a grain of salt, I don't speak korean so idk
The duke hasn't been cheating, even thou he didn't love her he stayed faithful, he is only pretending to be a gigolo so he can sleep with her because of his stupid pride. I actually didn't think it was that hard to understand this
Yea i kinda guessed from the fact they had the same hair color LOL