So I started reading this thread then fell into the Youtube labyrinth of Richard Hall and Peter Hyatt's videos, and now listening to the Maddie podcast.
I'm becoming pretty convinced that the McCann's were sedating their children (probably with something quite potent, the discovery of which would cause their career to be in peril). An antihistamine alone wouldn't knock out the twins to the extent they were. I think they were using opiates or benzos and using their medical knowledge to guide the dosing.
I think either Madeleine was overdosed, or, more likely (as it explains the blood in the apartment), she was underdosed, woke up, climbed up onto the sofa and Madeleine fell from a height while intoxicated and suffered a fatal injury, probably a head injury.
I think by the time they discovered her she was clearly beyond help and they knew that a postmortem would reveal drugs in her system.
The McCann's risked losing their children, their careers and their reputation, so they concealed her death and concocted the idea that she was abducted.
I found the statement analysis made by Peter Hyatt to be the most convincing evidence. I always found Gerry to be quite disingenuous, and he seems to have some narcissistic tendencies. I think he was a dominating character and Kate bent to his will. The statement analysis crystallised this for me.