If we work under the assumption of sabotage in their ranks, you could also put Bunny-hime in as a suspect because her panicked flailing broke one of the stones, potentially locking them in there. She may have had a human subordinate activate the barrier once they were all within. She's a decent enough actress to pull off that little hissy fit performance.

However, I felt that her panic was genuine. Being trapped kind of ruins a plan to gain fame.

I agree that if there isn't another layer to this and the 7th really is just a traitor, there is a decent likelihood that one of them could even be a fiend now that we've seen they can assume human form. In that case, with each one eventually demonstrating a supernatural feat, if one of them refuses to use their vaunted Saint power, they'd be the prime suspect for an imposter. In my mind, this removes suspicion on the frog girl, since she was ready to kill Flamie in the middle of the temple using her power until she was stopped. Assuming traitors, I'd put cat-boy and the mountain priestess as the lead suspects for an imposter-traitor.

I suppose we don't really have enough info to start to guess at who the fake would be if they really are a traitor. I just like the idea that there isn't a traitor and the imposter's desire to be a hero are genuine.