I already gave two plausible and logical reasons for opening the window which you either missed or ignored. No forensic evidence found neither proves nor disproves any particular theory.
Reasons which seem logical and plausible to you don't necessarily seem so to others.
http://miscarriageofjustice.co/index.php?topic=12172.msg659765#msg659765"liaise with an accomplice" suggests an accomplice was hanging around outside. If I was an abductor I would find it much more logical to open the front door if I needed to liaise with an accomplice. As it was recessed it would be less noticeable, reaching it would have been easier and it wouldn't have made any noise.
"check that the street was empty before leaving vis the front door." Peeping out of the door would be better than raising noisy shutters, it's easier to access and there's no risk of attracting attention by curtains blowing in the breeze, giving time to get away.
For those reasons I don't think an abductor would have seen it as either logical or plausible to pick his way past the cots and open the curtains, window and shutters. Opening the door would have been better; no risk of wakening the three children by opening the window and communicating with an accomplice and/or easy to exit once the child was picked up.