Obv it’s going to be the prince. They’ve forced it even before introducing the crossdressing prince. (In case you forgot: the mutual agreement that one person from the Veleotia line must marry into the royal family every other century or something that they talked about in the beginning of the manhwa)
I just read the chapter again. The condition was that the family must take a noble related to the imperial family, not "from" the imperial family.
Like say the family of the secretary.
Which is why it makes sense later on that the Duke used that to discourage the daughter of thinking of the prince as a possible marriage partner since he does not want her to marry. And she then said he will not be a part of the family once he inherits his father's land.
By then he would be "related" to the imperial family, but not a part of it

I would like it if there's no romance on Leo's life , but then it is what it is.