Well, Maya is not some dumb girl on the street. Why would she allow Tatsuya to marry into Mayumi's family
, and even if Maya couldn't order him around, what motive does Miyuki have to allow the marriage? Just because Miyuki is just tolerant of her clan, doesn't mean she is going to weaken them on purpose. It would go completely against her agenda to weaken her own clan seeing as she is gonna be the next heir... And how is Tatsuya gonna accept the proposal? He can't just make these decisions just cause he wanted to. Even then, marrying into Saegusa = less time he can protect Miyuki. There are agendas clashing all over the place.
And Koichi isn't gonna let Mayumi marry into another family and let her drop her name so.....