All hail the king who layeth the hammer on his idiot son.
Just for clarification, the Head Court Mage already knew that Sei is the Saintess because of these things. 1. He could appraise Aira’s status. And her status doesn’t have the Saintess title. 2. Aira’s magic is just like everyone else. It is barely explained in manga but the Saintess’s magic is unique, Sei’s magic has that unique aspect, so it was easier to assume Sei is the Saintess. 3. The subjugation that Sei went on didn’t explain well but the reason everyone treated her as if she’s the Saintess already is because she purified the miasma swamp, the monsters in the area, and the miasma in the area basically making that forest safe. Also her presence alone has an effect. 4. What threw off the whole investigation was Sei mentioning that in stories from her world that when characters go to another world, they often have abilities that are different from the populous. If the Court Head Mage did not go into a coma from the summoning ritual. He would have already determined that Sei is the Saintess from her odd already high level and her unique magic.
For everyone worried about the Second Prince being an issue, he isn’t. The Second Prince takes a more hands off approach and lets Aira be in Liz’s care since it makes more sense for Aira to be among girls her age. He simply lets her do things and will only intervene if something happens. And so far nothing bad has happened, so he’s just waiting and getting reports. In short Second Prince is maaaaaaany times smarter than his older Idiot Brother.
Also if you’re wondering how Captain Hawk and the King arrived. One of the peanut gallery ran off to get them because Sei got noticed. And was very lucky to run into them.