Raising the shutter would not secure entry if the window was originally closed. This woud tend to support the argument that the intruder entered the apartment by either the main door or the patio door.
Did you know that a maintenance man repaired one of the shutter pullcords in another bedroom just days before the abduction? He needed the apartment key in order to do this repair.
I think we all accept the possibility - even probability - of spare/duplicate keys. Where I have an issue with this is that it wasn't discussed as a possibility early on. I'm sure the police investigated the likelihood and it appears they never really entertained it as that significant based on the lack of coverage on the subject in official or semi-official sources. It only seems to be treated as a real possibility on what could be classified as "pro" websites and it's easy to understand why.
It would have been very easy in the early days for any of the OC staff, the police, the T9 themselves to say, "there's a distinct possibility that someone with a spare key may have been involved" and yet it's just not mentioned... instead we have broken/jemmied shutters and/or patio doors.
Only once that all starts running out of steam - and as I've said before, fairly early in 2009 from memory - does it start to gather momentum as a "theory" to explain how the comings and goings could have been achieved.