Interesting post.
You mention routine. In at least one of the Rogs it was recorded that Kate and Gerry were the most punctilious of the couples regarding their checking routine.
The criminals you mention did a recon ... "They would rarely enter an occupied house unless they had additional knowledge of the occupants." in Luz a recon would have supplied the knowledge of where the parents were ... knew the routine ... and knew there was no adult presence in the apartment.
Lifting Madeleine would have taken seconds ... particularly if she was being handed to an accomplice via the window. The shutter takes only seconds to raise.
Let's assume an abduction took place.
If the patio door was the access point, it would also be the egress point, as if it was suitable to enter unseen, in the gloom and in good cover, it would serve the same purpose coming out.
If the front door opens from the inside without a key (hence the question), they would unlock or even open it.
I can't think of a reason for opening a window on the same elevation as the (edit) front door, apart from if it didn't unlock from the inside. But if we assume that this abduction would have took seconds, as you suggest, and that they knew they had a clean run, there would be no messing about with any of the other egress points; it would be in through the unlocked door, grab and dash. No messing about - in and out, down the stairs and in to the night.
So why is the window open?
Did they leave any lights on?