Beth believes Natalee was at the vdS home at 3:00 am. I wondering how that plays into this.
I was reading a tin foil theory at RWV that junior and the brothers may have been on a mission for dear ol' dad. Perhaps dad was the one looking for a little action that night. Maybe that's why JvdS tells mom if she was home he would not have "thought about it," (sneaking out).
It's late, I'll ponder that thought more after some sleep.
Your post about the boys maybe involved with the father in getting a girl has reminded me of a tickle site, when I was following kids and their friends around, where a girl talked about "liking Joran's father". It was on a connection to the brother's site. I remember noting it but not making much of it other than the girl, a friend of the younger brother, Valentijn, knew the father and commented on him, it surprised me.
Adding another page to my tinfoil had theory ...
Remember that witness that saw some struggle on the beach, two men and a woman?
Perhaps the Kalpoe brothers dropped JvdS at the beach, later dad picked up Natalee and JvdS taking them back to the home.
That would explain how Natalee got to the home, if indeed true, and why LE seized both vdS vehicles.