I also thought like you but later on it's obvious that Hela isn't the demon king. It's obvious that she's the true saint but it was also obvious that she was working with someone evil. That's why he didn't trust her even then. In their 1st life he didn't thought that Adeline was the demon king he only wanted to protect her because she didn't have any powers but now in their 2nd she has some powers and they're detectable on her and similar that to the demon king, that's why. At least that's my conclusion. Idk
Because Adeline mana is on the same level like his mana and she can do think without spells... that's why he suspected her
Why doesn't he suspect that Hela is the Demon King? Why does he only suspect Adeline? That makes no sense to me. He knows she's an enemy, but won't take it a step further