I don't think anyone would describe me as a linear thinker, but anyway. Just to set the ball rolling in the spirit of Alice's thread... And this is just off the top of my head before this thread degenerates....
Burglary gone wrong.Nicks and duplicates key (possibly several while he's at it). Could be former employee, or knows / recognises someone who is e.g., maintenance or cleaning staff. Staff could keep keys with them over lunchtime in a jacket that gets hanged on a coat rack over lunch - nicks and duplicates some and puts them back in jacket.
Could have an accomplice. He or they case the village and work out the general routine of various tourists. They park near 5A, notice it's all quiet, but occupants are likely to have gone out for dinner (too early for bedtime). One enters via key, heads towards bedroom or living room, but M wakes up and is about to scream the place down. He puts his hand over her mouth to stop her, but accidentally smothers her. The choice is to run out and leave or to grab her (sentence is far heavier in the event of injury or death).
In a moment of panic, he opens the window and lifts the blind, either to signal to an accomplice, or possibly more likely because someone could work out that he'd got a key and decides to make it look as if he'd broken in via the window. Grabs her and leaves in the car.
The only thing I find odd is that the front door would have been shut again. Unless it could have slammed, or if there was a reason to avoid anyone guessing that he'd had a kesy. (I say he, but it could have been a woman or more than one person).
If he'd picked the lock (i.e., without a key), I can't think of a logical reason to open the window / shutter.
In the event of a
Smellyman high on drugs or suffering from some kind of dangerous psychiatric illnness (sporadic or not), , I find it hard to imagine what any thought processes may have been. Could someone in some kind of "altered" state of mind be lucid enough to lift up the shutter from the outside and get in that way? If so, I would have thought that there would have been more traces in the window area. Lucid enough to pick a lock? Wander in via the terrace side? But then close the gates? And wander out via the front door and think to close it?
I tried to find articles about the sexual assaults, and the one that seems to give the most details (accurate or not) is from the Guardian.
https://www.theguardian.com/uk-news/2014/mar/19/madeleine-mccann-police-intruder-girls-algarve