I am hearing more and more that Karr's wife has said she has no love lost for John, but knows he was with her at every Christmas when her kids were little. This kind of fact is one that cannot be overcomed or proven to be a lie. I'm starting to think this is not the right guy and just a real guilty liar.
I'm just going to wait for the DNA. It may not be him. What I find interesting about this thread though, is the analyzing of the psychology behind the perpetrator. I think there is a lot of insight. I can see how elements of this crime do fit Karr's personality given what we know. But like you said, they have to show he WAS there.