My theory changes daily lol, to me it is equally plausible that she left of her own accorded as it is that she was scammed by a man.
I believe it was her on the flight back to Aus because of the handwriting on the arrival card and the fact the police did contact the bank, the bank does not take those types of calls in regards to fraudulent use of a bank account lightly, they would have been thorough in their own investigation, it wasn't a small amount of money and the bank is liable to the account holder, the bank would have had to replace that money if Marion turned up and it wasn't her whom at removed it. So I do believe it was her.
It was a shame Sally went the bank route when her concerns were raised, I would probably have spent my time tracking down the travel agent and the storage people as they are not so legally bound with giving out information, but I have never been in Sallys position so whom am I too judge or give advise.
We have been through so many theory's have a look at the current one "Goran Markovic" and see what he did to an Australian lady, you will find it online at realscams.com her story could be exactly what happened to Marion.