I don't agree with some aspects of the above theory but the gist of it, I agree that possibly the reason Marion has never been seen again is because unfortunately for her, she did challenge RB in a way that he had not foreseen or expected because she was less vulnerable.
Maybe she had started out totally compliant and even changed her name to the frankly ludicrous Florabella Natalia Ramakel, as, IMO, doubtless firmly instructed to do so but she threw a spanner in the works keeping 'Marion'.
Also my belief is that as a teacher who had worked with youngsters for so many years, she was well used to seeing through and managing fibs, fabrications, temper tantrums, and all forms of childish outbursts as well as having a good understanding of what interpersonal bullying and intimidation are - these are issues any teacher would be aware of, professionally and experientially. As well as keeping paperwork in good order and dotting the 'i's, crossing the 't's and following rules to the letter, plus having a belief in 'the insitution', hierarchies, and forms of authority.
I suspect for those reasons that although she was led a merry dance, she started to see something wasn't real and true and due to her personality, she wasn't the sort that was going to be menaced by any man. She probably also wasn't the sort to bend rules to suit herself or agree to inappropriate conduct where it could be 'against the rules'.
Also, I think if she realised RB was scamming her, if it's true she was a bit of a 'firecracker' I think she'd have been indignant with rage and ready to alert all form of authorities and tell her family, not fold into a suicidal despair. Only the people who know her best would be able to comment on that.
JMO MOO