@JLZ - I married in England a couple of years after Marion's disappearance and I just don't believe the "fake wedding" theory. Government bureaucracy in the UK is pretty inflexible as anyone who has lived here, had children here or got married here will know. You simply couldn't rock up to the register office and give fake names, and get married. We needed all the sort of ID you'd expect to have to provide - driving licence, passport, something with address to prove where we lived.
So in order for Marion to have had a wedding in the UK just days after she'd arrived, she would have had to change her name again, get new documents issued in that name, convince a registrar that it was kosher, and also get them to turn a blind eye to the fact that she wasn't legally allowed to marry in the UK on a tourist visa, and hadn't been in the country long enough to get married? Not buying it at all, I'm afraid.
She said she was married on her return to Australia, but she also said a lot of other things we know are not true. She may have been "married" unofficially in some quasi-religious ceremony or considered herself married in a "we're committed" sense, who knows. Or was just trying to cover her tracks further by arriving in Aus as a married woman called Florabella rather than a single/divorced woman called Marion. Who knows.
The whole speculation about spies and espionage is just too far-fetched. This is 1997 we're talking about, the Berlin wall had fallen, the USSR had disintegrated. I also agree that many people with German surnames changed them in the first half of the 20th century, even the Royal Family adopted Windsor rather than Saxe-Coburg-Gotha.