" Which brings me back to what Beth saw or heard or realized when she & Greta were in the VDS house...""
Yes, what Beth heard or as you said saw too, is very important. Beth seemed to be very confident, about what she observed at the VDS house.
This is a stretch, especially weeks after the disappearance, but maybe Beth smelled Natalee's perfume there? My daughters all used distinctive scents, and when walking into a room, I could tell which one had passed through. (Certainly not after days, though). One always wore a musk; another, lavender; the third one, her mother's Chanel.