I think there is a possibility(but not a probability) Madeleine could have opened the window if it was unlocked and IMO she would have done that before attempting to do anything to the shutters. The lack of identifiable fingerprints found does not mean she did not touch the handle or lateral side of the pane's frame - after all, no prints were found belonging to the female cleaner.
From a child's POV she may have thought that by opening the window the shutters would have opened at the same time. After realising that the shutters didn't open by that method Madeleine would have had to establish how to do that. The covers on the bed under the window were not neat & tidy so that could have disguised the fact she'd played around with the window & curtains whilst standing on the bed.
However, there is no true explanation as to how the net curtains arrived in the position shown in the crime scene photos - nets are difficult to move when they bunch up on a rail or cable.
I do not think Kate was mistaken about the open window. The sight of it open combined with the raised shutters prompted the thoughts of an intruder followed by the terror in realising Madeleine was no longer in the apartment.
In the case of Isabel Celis (Tuscon 2012), another girl who was apparently abducted from her bedroom, the window in her room was discovered to be open & the exterior shutter broken & on the ground outside. Despite the fact that CCTV cameras nearby could monitor the bedroom window, no-one was seen entering or leaving via the window. This was due to the fact that the cameras were motion-activated so whoever opened the window & broke the shutter did so from inside the property - probably in an attempt to hide a more legitimate access means.
Were both net curtains pulled back, or only the one in front of the open window?
I ask this because it seems to me that it would be difficult for anyone standing outside the building to push back the net curtain that was behind the closed portion of the window.