the novel translation stopped so i can't be entirely positive, but the reason the new hot man is cozying up to rashta is because Henly asked him to, in order to a) force the emperor to actually choose between Rasta and Navier, so that Henly can run off with Navier once the emperor inevitably decides to be a big dumb idiot, and b) as a means of weakening the empire by giving it a weak empress and a simp emperor, so that Henly can eventually invade and conquer the empire. He later says something along the lines of "It's wrong to lower an empress to a queen, so obviously I have to create a new empire for her" along this same sentiment.
Admittedly, the new boy working behind the scenes to cause a divorce between you and your partner so they can swoop in and save the day is very sketchy and quite a red flag at face value, but given the context of this story I don't think it's too bad