I'm not against the idea of this being a ship. The age gap isn't that great, Nina is actually an adult on the inside anyway, and it's not like the king is pushing her to do anything she's not ready for, nor does he appear to be grooming her in some kind of creepy way.
I think the real issue is that Nina feels her position of relative powerlessness means she won't feel she has a choice in the matter. She also struggled to express those feelings. There does seem to be a mutual attraction on some level, but since Nina doesn't wish to get involved in situations she doesn't have control over, she's unwilling to face those feelings. To me, it will be more interesting if the story forces Nina to confront those feelings and see what choices she makes, rather than presenting a more convenient male lead that she can comfortably stay the same with.