The royal symbol was the only proof of the heir's identity, no one even knew the guy's face, so with the prince dead there's no one left that knows who the real guy is, and with the symbol in his hand he is handed an entire country to rule over on a silver platter, what kind of motivation could he have not to take it?
Plenty. Fear of responsibility, the notion that one day he might end up meeting a foreign diplomat that would expose him, potential to get embroiled in a game of thrones with the nobility that he is ill suited to due to a lack of experience with that world, the chance to hilariously fail a whole country due to again a lack of experience with leadership. etc. etc. If anything it's very brave (or foolish) from him to have accepted it when you think about it. I am not sure that I would. This said, given his behavior later in the plot we see that he has things in hand. But that's him.
This all being said, my old question stands. I only just now ran into Dia. I finished her quest up until the point where she sits in my room with her sister happy and saying shell stick with her forever. Cured her little problem and everything. Now the manual says I need to go talk to someone in the new shrine to unlock her sex scenes. But talking to the people there yields nothing. All I get is generic dialogue about how she is a good girl.
What am I missing? Is there a hidden condition or something?