IF it is Dylan in there, and IF MR put him there, then surely MR isn't going to sit around and wait till the divers recover him?
Apologies for the graphic nature of the next bit .....
I mean, the conditions must be perfect for preserving forensic evidence (injuries, proof of drowning/proof of a different cause of death, food in his stomach etc). If its Dylan and he has cereal in his stomach, then MR's story is true as far as Dylan was alive, at the house, and ate cereal on the Monday morning. However, I guess there is the chance that something could have happened AFTER MR's return from his chores, which I don't think has been suggested yet?
How bizarre would it be if the stories of the dead phone, no internet access, Dylan sleeping in and not contacting anyone were true ..... and the actual answer was something nobody had considered. Like MR coming back and saying "Sorry, but I'm really not up for that drive now, I've been on the go all day" and Dylan getting angry having waited all morning? All just brainstorming, not suggesting it happened.
However, if he had McD's in his stomach, then the whole Monday morning story is a pack of lies and can be proven to be. What I'm trying to say is, if its Dylan and if Mark is responsible, why would he be calmly waiting for the truth to come out, if he is guilty. Because its no secret that there is very likely a body waiting to be found in that lake, and no one else appears to be missing.
So, if MR sticks around as he has done so far, surely this could actually indicate that he has done nothing wrong at all.