There's no need to establish that he lies.
What we need is for his followers to admit it to themselves, and that it does matter. And that is going to be much, much harder.
No, I don't think that's very likely. As in all cults, what keeps them going is the fear the faithful have that they'll have to admit to themselves they were conned. The question we need to be asking is, "What will the Kool-Aid moment look like?"