Yep I'm trying to figure this out myself.
So, with JO he wanted her out of the way whilst he 'took' her house.
Working on the same method, he wanted Marion out of the way whilst he 'took' her money.
So, maybe it's blindingly obvious?
With JO, she did not follow instruction to take a flight at a later date, and thwarted RB by coming home early. Albeit accidentally. Insofar as she hadn't seen through him, despite her cousin (I think it was) had suspicions, she did not.
With Marion, she did not thwart his plan, accidentally or on purpose, she did not return early. The money was taken, nobody knows by whom, and nobody has ever seen Marion again. We do not have any proof she returned to Aus.
Maybe that's the logic. His plan worked. He came back and took her money whilst someone - who? - 'dealt with' Marion in the UK.
The clue to unpicking that would be examining / scrutinising EXACTLY the minutiae of where exactly did RB leave JO in the UK? what did he tell her to do? where did he tell her to go? what instructions did he leave her with?
Was RB planning to leave JO alone in the UK and totally disappear, leaving her alive and well but somewhat stranded on the other side of the world? Or did he have some fate 'arranged' for her? If so, what? Did he suggest JO some contacts or places to stay or companions or any type of instruction whatsoever? Because those are the clues.
Did RB think he could strand Marion in the UK, drain her bank account, and just disappear, leaving her alive and well and able to tell the tale, return to Aus, report to the police and bank detectives, all of whom could try to track him down somehow and then she could identify him? I suspect not. How realistic would that be?
Therefore, he possibly has at least one accomplice. A female accomplice, or a couple, who 'dealt with' Marion in the UK and then the female travelled on her passport? The Dutch Dentist and 'Maz'?
JO is a very very lucky to be alive woman.
Also, something that crosses my mind whilst pondering all this - as RB has possibly simultaneously held numerous identities and passports (fake or real), we don't know if it is even him who travelled back into Aus on his own passport in the July, if you see what I mean. Even that cannot be truly depended upon as a definite fact.