Hey, I have the same opinion about Maura being the Seventh. She has the key, so it's very possible that she had already entered the temple before. Although we don't know how she/anyone else activated the barrier.
As for the whole characters... Adlet is not the culprit, we know that because we saw him getting the Flower sign - unless we're dealing with unreliable narrator. Nach is innocent, in my opinion, because she's seem too naive and I don't think she would dare to mean any harm to anyone. I also believe Flamie and Chamot are not the culprit too. Chamo is too chaotic and I don't think she could set up a trap to frame someone - she doesn't look that smart to begin with. I have divided opinion about Nach's knight-whathisname-. I don't think he's the one, but I thought it could be possible that he just want to be with Nach but he wasn't chosen so he made a fake tattoo, and he seems to be jealous that Nach put so much trust to Adlet. Hans is the most suspicious one, but you know in murder mystery, the most suspicious person can't be the culprit (lol). That leaves Maura.
Lately Rokka has been full with backstories and such, but it sure is weird that it changed the genre from action adventure to mystery solving. It didn't bore me so much this far. But we're already at the 7th episode and still stuck in the same place. We're not getting anywhere near the Demon Lord.