Your theory makes little 'real world' sense.
Irrespective of the timings and apparent plot holes, as Bloggsy has eluded to, you seem incapable of factoring in the human and emotional aspects. While I agree that desperate people resort to desperate measures, and have stated as such in the past, it is inconceivable that a relatively disparate group of young adults would conspire in such a way. It would be unprecedented in all of modern criminality. Can you provide any criminal undertaking as a parallel?
I don't believe the rest of the group were involved. Maybe David knew something was up & amended his statements accordingly but that's about all. Anyway. The McCanns are both clinicians they would have worked in emergency departments during their training as doctors. They would have been involved in clinical, emotionless high stress high pressure procedures, possibly witnessed major trauma & death, so I don't find it beyond the realms of possibility that they could approach a problem without too much emotion if needed. Either Russell or Matt, can't remember which, said that usually stoic & composed Mr Mccann was rolling around on the floor. But that would have been after he did what he needed to do imo.