I think it's better if he swap the soul back, then seize him and make him the king... Then boom, instant win, (The End) he ruled the whole upper realm. (Da** my wisdom scare me sometimes)
@Kunohana Nah, it would just take the prince to try to silence MC due the indignity and he would get trouble for no good reason. Even the best case scenario is the prince becomes a half-follower, and surely the fate of thw wolrd is something that would make him eventually turn coats.