It makes so much sense for Peter to be the one holding the Rushmore idol.
Comfortable doing the mutiny, check (hey this tell applies to me too but only Writer knows I volunteered)
Comfortable throwing away his double vote on a brash attempt to flip a vote after the mutiny, check - psychologically that heavily implies he's got more up his sleeve to give up the voting advantage that fast
Participated in the Train challenge in such a way that there were time gaps in his posting, check - this is where he would have been making the required Heist posts in his confessional
Buckley could feasibly have been making the posts in his confessional but he was apparently playing entirely from his phone for the first several rounds of the game, which would make it extremely difficult to pull off the Heist. Margot didn't have any posting gaps in the challenge. Steve and Max are dead and gone.
I see you, Peter Peter Idol Eater.
After re-reading my Heist instructions, I am now wondering if the "tape on the nose" thing indicates that this is something that would have been visible to the player with the Binoculars if they were using their item correctly. I'm going to be keeping an eye out for that.