Part of me wonders, (I'm sure this has been discussed before, long before I joined the forum so apologies for any repeats), if Marion was being blackmailed.
It doesn't fit all elements of the case, I admit, but if she went to the UK to try to escape from a perceived threat, that might explain the name change, the secrecy, the weird meetings and the shadiness about her identity.
If, somehow, someone then got in touch with her in the UK, making clear to her that they would still release information about her regardless of her location, perhaps she flew back to Aus, withdrawing the money to pay off the blackmailer.
If it was information that she desperately didn't want anyone to know, she might have chosen to keep things secret from her family, getting herself into a deeper mess than she ever anticipated.
Again, just speculating, but if this was the case, I can see that this might somehow link to what was going on at TSS, hence the strange insistence in her postcards about the scales, to which she makes a comment that I have always found jarring that is something along the lines of "so he doesn't have more ammunition against me" (I think in ref. to Luke Glover (please correct me if I'm wrong.)
This, for me, is more plausible for many reasons than falling foul to a con-artist willing to travel back and forth to enact some EXTREMELY lengthy, risky, convoluted plan.
I have no idea what could lead to her being blackmailed, but people will go to great lengths to keep things from coming to light if they are ashamed of something, regardless of how monumental it seems to others. We know Marion cared deeply about her reputation, so this fits with some of the elements we understand about her character.
I'd love to hear what everyone else's thoughts are on this.