Why? He's being super indecisive so she's pretending to leave to give him the push he needs to strengthen his conviction about his feelings. He already realized he loved her that night, but making him worry about her has made him introspective and consider her feelings. It's killing two birds with one stone as well, since the plan is to clue him in on that he needs to make everyone know they're marrying for love.
Lol, that's what you call "Plan". Asking him directly would look like SHE pushed the duke to do it and will not just be good in the eyes of their social circles but also, NO DEFINITE ASSURANCE OF A HAPPY FUTURE. That plan was not just to completely satisfy Adele's wants and needs but also to weed out the rumors rotating in the whole empire. ( ̄へ ̄)
She is selfish now but this is still the same Adele that I know from the very start. Killing two or three birds in one stone. This drama and her selfishness doesn't merit only herself but also the political status of both of them and the Crowned Prince who is always behind their backs. The duke might be struggling now but still, Adele also thinks the good effects of all of this in the future.
After reading these two chapters all i have to say is: i hate adele.