I think the overall reason as of why robin's fiancee became the empress is because that was what robin wants, he want his fiancee to be on her full potential because he knows that she can be more. And I think Yan and Robin's promises was to make Yan the emperor and make his fiancee the empress and yan wanys to make the knight stay by his side. If you don't understand, long story-short robin is not a prince so he can't compete to be the empeeor and make his fiancee the empress. Yan didn't care about the throne but he agreed because robin's condition is he'll make sure that Ian stay's by yan's side.
Oh noooooo, there’s noooooo way that the emperor made your brother’s fiancé his empress so that you guys could be together and the brother who may or may not actually be in prison so he can work from the shadows or something could be with his wife so no nobles would look down on two guys who can’t have an aire and the succession process doesn’t have to be complicated. There’s nooooo way that kid with the exact same hair shade as our mc is actually....the brother’s kid?!??!?
Honestly these are my predictions so I hope I’m kind of wrong