I know that I shouldn’t be focusing on a statement that has been declared a mistake, but this scenario is the first that actually makes sense to me in this case. I’m building on everyone’s ideas here, but what if Marion received a diagnosis, decided to seek alternative care and used the ruse of a prolonged trip to the UK to receive this treatment without her family trying to convince her otherwise? She seems like such a spiritual lady, it seems entirely plausible to me that she could be interested in alternative therapies. Maybe she always planned to come back to Australia soon after a brief holiday in the UK, or maybe the treatment was supposed to happen in the UK but she had to come back for some other pressing business. If she died in the care of a alternative care provider/healer type person, or was being cared for in some kind of cult, they couldn’t really report her death without facing possible investigations or charges themselves, could they? The Twelve Tribes baby deaths are an example of this. This particular tribe didn’t trust or use standard medical care and as a result, the death of at least one baby was not reported and they were buried, presumably by the tribespeople, with no authorities ever alerted.
Some details about the alternative medical care of The Twelve Tribes:
What drove religious elder to leave controversial cult
The details of the current investigation into the deaths of babies:
Police 'find body of at least one baby' at Twelve Tribes religious sect headquarters | Daily Mail Online
I’m not saying that this is 100% what happened to Marion and that she joined the Twelve Tribes. It could have been a different organisation or individual treating her. Just that IMO, this is a plausible scenario where all the puzzle pieces seem to fit. The strange behaviour before she left, the brief trip to the UK, selling her house, changing her name (it sounds like such an alternative/cult-like name), quitting her job, the bank withdrawals in a strange location (maybe she needed more money than she anticipated?) and the effort she went to so that it would be hard to find her. The pieces just fit when thinking about this theory.