So....
After Madeleine disappeared the shutter was ... what? Somewhat lowered until just open, at which point Gerry could raise it? That I can buy except no one got in or out with the shutter in the position of the PJ photo.
However, Dianne could not raise the shutter. Was she weak or was the shutter fully closed? Or what?
Then the PJ photos show the shutter slightly open. Not enough for someone to get in/out, but not closed enough to beat Dianne.
We have a magic shutter going up and down that might or might not be able to be raised from the outside. (It could.)
I think Dianne was unable to lift it because it had jammed in that position because the interior cord had not been used.
When Kate first saw it raised she was able to look through the window to check outside, therefore it must have been fully raised.
Quite obviously the shutter should not have been touched but at that stage only the hysterical mother thought it was a crime scene. It was stupid to do so but I can appreciate why it was done. Although panicking and still unaware of the reality of the situation Gerry checked it out because he was at that stage unable to grasp that the shutter could be raised from outside .
It is only recently that the McCann sceptics after arguing for years have accepted it can be done.
It is intriguing that there were no prints lifted from that area either for Dianne or Gerry.
I do not think that entry was made to the apartment via the window although it remains an option given that was apparently the preferred entry point in burglaries in the building in the preceding weeks.
Heri gives a very plausible explanation
http://espacioexterior.blogspot.co.uk/2013/04/the-madeleine-mccann-abduction-janosch.html which has Madeleine approaching the window and being lifted through.
However, because no evidence of entry was apparent doesn't mean it didn't happen (there is no forensic evidence that Gerry and Dianne interfered with the shutter) and someone lithe enough to get in and out of windows could easily do it without leaving a trace.
So until we know better, I think it remains an option, perhaps it wasn't closed behind him for the simple reason of haste and his hands were holding the child.