Yup. The only part of this theory that I find confusing is *IF* this was the intention in going to the park, why take JJ? The answer to that would seem to be why would Alex or Alex and Lori take Tylee to Yellowstone and not bring JJ as well. Not bringing JJ would seem suspcious to Tylee. (How she's react to that, I don't have a clue.) But that means if this is what happened then they're going to have to insulate JJ from seeing too much or it might backfire. (Did that happen?) If the intent was to get rid of both of them why not take care of JJ at this same time?
I'm also curious if LE has possibly checked cameras between Rexburg and the entrance to Yellowstone to see if Alex, Lori, and JJ might have visited again sometime around the end of September. MOO