My theory for what it's worth (so nothing)
Parents gave the children something to help them sleep through the night, so parents could have a meal out with friends.
Something went wrong with the dose, I would guess not enough for Maddie, she wakes and is drowsy, falls and injures herself. She doesn't survive.
When she is found the parents panic, if it's discovered they are dosing up the kids it's goodbye to their medical careers, goodbye twins, goodbye nice, respectable lifestyle where people admire and look up to them.
It was reported in the first few days of Maddie going missing that a sports holdall was in the apartment belonging to Gerry, that bag is seen on holiday footage in the wardrobe but after "abduction" never seen again.
After discovery Maddie's remains are placed in the bag, bag stored somewhere outside of apartment. Police are alerted to her disappearance. Chaos breaks out, search begins.
The parents continue with their tennis lessons, rather than search for their daughter.
At some point the bag with remains are retrieved, put into boot of new hire car and disposed of (into the sea?, If a friend on the holiday did this we now have a motive for all remaining silent)
Fundraising begins in earnest, Pope and various celebs are dragged into meeting the McCann's.
A series of sightings, predators are alleged over the years, all conveniently dead, terminally ill or already in prison. No progress is made.
The questions I think need answering:
When was the last independent, verified sighting of Maddie? Not as reported by parents, friends or any signing in or out of kids club but an actual unrelated party that can say the time and day they last saw her? (Establish timeline)
Are there facilities at the tennis lessons for those attending to store personal items in a locker area? (Store bag and remains)
The cadaver dogs alerted to an area behind the couch in the apartment (the fatal fall?)
Also the bottom of the wardrobe (where the sports bag was spotted in the holiday video, where the remains were initially placed?) then the boot of the new hire car (remains moved and disposed of?)
The dogs used have worked extensively with law enforcement worldwide, the FBI have used them successfully, so why was Gerry so dismissive and smug when asked about them with the comment, "ask the dogs" and then laughing it off when questioned over why they alerted to blood and human remains. What about any of this is funny to him?