I think Maddie could have died after inadvertently consuming adult recreational drugs, in the possession of one of the group, and that the other little (Tanner) girl who also consumed some of the same drug, turned out to be violently sick, but she survived. I am 99.9% certain that this is what happened. It was an accident, but her death was obviously brought on by the consumption of these recreational drugs. That was / is why they got rid of her body, because one or other of the group had in their possession illegal adult recreational drugs, which would have had serious consequences for them (Criminally) legally and career wise...
If you read through the statements of group members there appears to have been a frantic period whilst GM was playing tennis on the evening before Maddies disappearance off the face of the earth, where one member of the group was wanting to know where Kate and Maddie was? This was because at that time somebody had become aware that the Tanner child had taken some of the recreational drug I am speaking about? The group member wanted to see if Maddie was having similar symptoms as the Tanner child, and that night Maddie died, and the Tanner child was very poorly, vomiting, sweating, hallucinating, etc., but she survived...
This is what the group have covered up...