2 days to go so I'll make my final predictions.
I still think The Yellow King is Maggie's father. He lives on the shore and is wealthy. It's also "right under Marty's nose".
Tuttles, Childress etc, are all his pawns/lightning rods to distract attention.
However he has no son, so the crown would fall to Maggie. She's also in on most of it, at least since she ****ed Rust. This successfully ended the partnership with Marty and the investigation.
And thusly when Maggie is gone, the crown would pass to her daughter Audrey. This explains her dolls, her drawings, and the fact that she had that crown she threw up in the tree that even had streamers on it like the crowns the girls wear in rape video. She was never sexually abused, she's simply being taught what her future is.
How Marty will react to this I have no idea. I suspect however that after the revelation, Marty may very well lose it and murder his ex-wife unwittingly making his daughter the one who now continues this horrible tradition. Rust I still think will be killed by the cult. I doubt it will be suicide, his line about contemplating Jesus and the crucifixion makes me think his death will serve a greater purpose like either trying to save Marty or allowing his corpse to be evidence to convice the cult etc.
All of this also explains the dissapearance of just one boy. All the girls missing makes sense since they are needed for the sexual ritual. So why one boy? A sexual ritual for Maggie?