I am left with two massive questions.
1. How did the prince-disguised-as-a-lady "show signs of [saintess] power"? Can magic be used to mimic saintess powers, even though they are explicitly two completely different things, and even fool an ancient demons?
2. What do you mean "Royal blood is the sole key that can interfere with the Saint's power"? The royal family has anti-saint blood!?! Is their blood literally unholy?!?