In the past few days I've read the brothers and possibly Joran have stated they thought Natalee would turn up in a crack house somewhere. If true that gives some validity to the kidnapping/sex slavery theory.
Also, in those situations, it's not unusual for the sellers or the handlers to break in the victim before shipping her off. The victims are primed through drugs and rape (Daddy and Joran??). Just a thought.
Very likely. The problem then becomes is Natalee seen as a liability if she's still alive.