I don't think the prince would have cheated on Yuri. All of the four MLs represent some problematic cliche ML trope and the prince is one of those cases of "The MC isn't like other girls, she's SPECIAL and DIFFERENT" and that's why the ML who has always treated women like disposable sex objects can be loyal and faithful to this not-like-other-girls-heroine. She says in the beginning that the Prince, her brother, and the werewolf all treat Yuri very well, but they treat Melissa like trash and that's a problem, because someone who treats people differently just based on who they are isn't a good person. Other than Yuri, The Prince didn't think of the woman around him as people with their own feelings and personalities and so could easily use and discard them. The prince probably treated Yuri well, but only because she was the novel heroine and therefore SPECIAL from other girls.
Honestly tho like out of the four options for Yuri in her past not now I think the wolf would have been the better option? Idk but I think he'd be the one I'm most comfortable with? I mean Melissa's brother is a freaking stalker so he'd be watching me all the time so that's just a no for me 2. The rich dude with green hair is just a freaking psycho that would probably lock me up in a cage somewhere in his dungeon so that's also a huge no thank you and 3rd the prince is just scum he's slowly getting better now that Melissa is like teaching him how to grow up but if he just stayed the same because Melissa hadn't transferred into the story he's just a cheater that found a really pretty girl and wants to marry her and he'll eventually probably cheat on her once he finds someone else ya know more attractive x-x idk but I feel like the wolf could have grown in that relationship with Yuri if she didn't just let him do whatever at least that would have been my choice if I was her I also kind of feel like her brother would have been a decent option depending on if you're okay with him constantly watching over you I don't think he'd be the type to like flip out if you start talking to someone else or try to kill the person you're talking to LMAO but I'm genuinely not comfortable with the thought of eyes constantly on me 24/7