Haven't thought of this case in a year or so, but I have a new theory. It struck me reading the brilliant Time Line on page 11 which suddenly made Marion make sense to me.
FWIW. . . .
Spring 1997 - Marion has an affair she keeps from family, friends, colleagues. Maybe he's married, maybe he's abuse. Marion knows what the family thinks of her infatuations and failed marriages, so she is loathe to open herself to criticism. She is reassured that Sally is marrying, and Owen is old enough to take care of himself. The rocky relationship gets rockier. Sally sees Marion with this man.
early Summer 1997 - Fed up and wanting to extricate herself or to punish the lover, Marion ends the affair and to make sure she doesn't give in to him, she plans to leave Australia. She changes her name impetuously, sells her house, and leaves the man behind.
UK Summer 1997 - Marion gets what she wants. The man begs her to come back. Marion resists but then capitulates. Last thing she does is call Sally, maybe even wistfully. She can tell Sally everything once the relationship is concrete and things work out.
August 1997 - Marion returns to Australia. The man turns up the pressure either emotionally or criminally, and Marion withdraws her money. She either can't contact her family or she is too humiliated that her grand tour never happened, and she still hopes the man will make good his promises. She's trapped.
Late 1997 - By the end of the year, Marion is dead. The man makes off with her money. His wife or accomplice phones police about the body.
What's different for me about this theory is that UK and Europe are innocent. The danger was all in Australia. What if Marion was trying to get away but then gave in and returned?
The Time Line is brilliant. Now I'll go back and catch up. Thanks for listening!