I am not sure on the "he killed Marion to keep her from going to authorities or police" so far he has shown very little care if the police know or his family, he uses the name DH to fraud women in Australia and oversea's while telling them he lives in Aus, the name DH is easily traced in Australia, he tried to scam a "friends" wife JO, who could easily identify him, asks a Neighbour if they know anyone who can make a press to make fake coins, he has a Neighbour sign a passport photo is another name, no shame is doctoring coins in front of people, tells people he is claiming a disability allowance when clearly he doesn't look like he needs one and travels the world.
He is hiding in plain sight and is so confident in his ability to weasel out of anything, and clearly has, he doesn't seem to need to get rid of any witnesses
If he did kill her I don't think it was because she threatened the police or going to his family, I think she would have been in a long line of people who has probably door stopped him and his family, pretty sure they are used to it ... that's why they need to fess up