The answer to " Could an intruder have opened the shutter and climbed in the window?" is, of course he/she could.
We have watched numerous videos demonstrating the mechanics of this type of shutter being opened and closed from inside and outside apartments. It has been generally agreed the noise made would be ignored because it would be such a normal thing to hear.
Similarly there are many demonstrations of people entering via this route and demonstrations of how a child could easily be either lifted out, or passed out to an accomplice.
The question is why would an abductor use the window, apart for observation, when he/she had a perfectly good very secluded and private door to use?
Why risk drawing the attention of a passer by entering or leaving a property by a window, when no-one would have given a second glance to a person entering or leaving by the door, or who by the secluded nature of the doorway could have melted into the shadows till potential observers had passed?
IMO it was not a burglary gone wrong, but a well planned abduction attempt which for some reason was almost derailed.
IMO this is why we have accounts of Smithman and Tannerman out in the street, I don't think that was part of the plan, therefore I think the shutter being raised was part of that last minute upset to the plan.