Responses

'King' is simply a title. I assume there's been a bit of mistranslation in the middle. So he's probably the son of the Emperor and the brother of the Crown Prince and he has the title of 'Devil King'.
I'm not sure about the Empress part. Considering their relationship, it's possible the Empress is his step-mother but then again it doesn't make sense if he's the son of Empress Dowager, so I assume there's a lost-in-translation moment here haha.
I’d love to know what on earth is going on. First they call him the fourth prince, right hand man to the king and brother to the crowned prince; next they call him the king. First they say that his mother is the Empress Dowager (I.e. mother of the King), but next scene they say that his mother is the Empress. What’s going on?
Also, have they even been married? I saw no marriage scene but they’re talking as if they’re already married.