cheating

But they are all single
The crown prince has candidates, but all in all single

I was thinking the same thing but it's different he has candidates and he told his father he wanted to be with her so no cheating here

yep
it is just that the translation made it seem that that girl was his fiancee. But not once did he say nor she said it themselves, like they said, candidate. It just that she seems the type that, in all fairness, finds it outrageous that someone from a lower status would become the crown princess when she, the daughter of a Duke and had to suffer all years of training to not be chosen because of a country bumpkin. But at the same time, if the crown prince choose the protagonist and they do fall in love, it is just fair for them to admit defeat.
Some people are complaining that she is the heroines, but I am glad she is. Due to her past life, I want her to have a happy life with a good man here.
Besides, the heroines in the game could be either the protagonist or the daughter of the duke, in the game you could choose who to be. The maid isn't sure in which game they are set in.
Well, the novel goes like that, dunno the manga
But it is nice anyway,
For the heroines is a nice girl, and there are decent, single men around her.

In other words, she may be the villain without her knowledge ╮(￣▽￣)╭