Dude, I imagine the more she falls for his sweet words and kind acts the more she will push for the annulment. Her goal is dumb that way. She would act like she's sacrificing her own feelings for the one she loves to be happy...
I just wish she doesn't actually fall in love first. This would turn into a wish fulfilment story for manipulative pickup artists if that was the case.
Let's just hope that she'll learn about his rank climbing strats early. His older brother seems to have an idea at least, the rest of the family probably have an idea too... it's just that they're all thinking about their noble duties. (Which involves political marriage.)
The father is probably conflicted about sending his innocent daughter off too, but the suspicious guy is a super talented genius. Better to tie him in with marriage and control him using the rank as a reward (even though real life nobility politics isn't like that). I'm sure if MC actually puts her foot down, the father would annul it.
Also, this guy literally has backups for backups. If he didn't get the engagement he would probably go for the duke's daughter instead.
The author is definitely setting him up for a change of heart... but he was still a manipulative shit stain. I just hope he didn't go farther than giving out compliments to women in high positions. It would definitely suck if he was an actual playboy with how they act when his history with women or being a "wolf" was brought up.