So a child dies, the parents are allowed to keep the body but then someone steals it? You have a fertile imagination in my opinion.
I have the basic outline of how it could happen, but whether it did happen that way is practically impossible to prove. But if you were to say the cadaver odour in the apartment was due to this other cadaver, you would have to have a mechanism to get it in there, and later to get it out. And to have a method of getting Madeleine out and then for Madeleine to have been removed from the vicinity all for good reasons.
I don't imagine any event in this scenario being a truly criminal act. Maybe even the adoption of Madeleine could be justified, I don't know, but Kate was willing to forgive the abductor wasn't she? How could all that happen for an altruistic reason?
It could even be that the PJ are aware of the whole scenario but can't prove it. Solving the case depended entirely on someone making a confession (i.e. owning up to the non criminal acts).
One of the reasons I consider it possible that GA knows what happened is that IMO Eddie alerted twice in the dining room at the villa but neither are recorded as an alert. Then after that it was several days later that the hire car was taken for testing but only after the trip to Huelva was allowed to happen. Amaral has suggested (somehow) they were carrying a body on that trip, was he right?
But that makes it confusing for at times he says they got rid of the body by cremation. Was he also thinking of the possibility of two bodies?