Why do you permanently insist that any death & concealment scenario has to involve the entire holiday group?
It doesn't.
As far as I'm concerned the only viable plan would involve NO-ONE except the McCanns - because it's obvious it would be far easier and far safer to dispose of their daughters' body during the night and on their own - without even telling anyone else - let alone getting them to tell numerous huge lies and make untrue claims on their behalf about what happened that night.
The present different 'scenarios' being suggested would of necessity require everyone else being 'in the know' to some degree. However, it would also mean that all nine of them were severely mentally impaired - which is another reason why I believe Madeleine was abducted and no lies were told. But - as I've said before - that doesn't mean that everything they said was accurate - because memories are notoriously unreliable. Something which experienced policemen know all about and so would expect discrepancies to occur.