There seems to be plenty of evidence (lichen/sills/bed) that nobody has entered or exitted through the window. This leads to the only alternative being staging. And in the case of staging, you would not logically land on an abductor or burglar but more likely on one of the parents being involved. And if it involves the parents, I'd rather go for one (GM) than both being involved.
I am still bothered by two very strange anomalies in GM's earliest statements. One is that he claimed entering the front door when he checked the kids at 21:00 (that he later corrected) and the other is his peculiar focus on checking whether the shutters could be opened from the outside after the alarm was raised and panic broke out.
The first one is pretty strange given that they always entered through the patio door to check their children in the evenings. If GM however had found Maddie already dead, staged a kidnap by opening the window (avoiding fingerprints) and shutters and then used the front door to move her body out of the apartment, he had to explain why the always double-locked front door, suddenly was found unlocked. Him using a lie of having entered through it the night before is then the safest explanation to mitigate this problem in an official statement the day after, but something he could relax about later and say he went through the patio door. It is however a noticeable and remarkably inaccurate element in his victim statement of May 4th.
The second anomaly around checking the shutters, is the most likely action to focus on when you have to make sure that your staging is feasible. This could even have been his biggest concern whether the staging would be believable or not. Clearly KM (innocent in this scenario) fell for it and immediately assumed an abduction had happened and that message rapidly radiated through the T9 group, the press and was reinforced by the Tannerman sighting.
So, GM reporting the door wider open is untrue, however MO seeing the door wider open and the room being lighter are true, KM seeing the door wider open and it slamming when she tried to close it are also true.
It seems the simplest scenario possible, but of course the key question is where the body then was hidden and later removed to (e.g. in a sports bag and later dumped in a bin).