With the caveat abduction is a theory and not proven fact (and there are other theories - e.g. tracker dog trail indicating w&w) I would speculate as follows (briefly):
1. If the shutter was pretty much fully raised with the window and curtain open then I would speculate the following scenario:
a) the front door was double locked and the intruder did not have a key
b) MBM was the target
c) the intruder entered via the sliding patio door, I would have expected the patio door to be secured by an intruder
d) the intruder may have subdued MBM - e.g. binding arms/legs, etc.
e) the intruder had an accomplice
f) MBM was passed through the window to the accomplice, who took MBM away possibly without the intruder
g) the possibility is, given lack of fibre evidence on the window frame, that the intruder exited out through the sliding patio door, leaving the patio door unlocked
h) this may indicate the intruder was an insider who had to return to work
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2. If the shutter is was not disturbed much then I speculate that the exit point was the front door, but there was a desire to "check the coast is clear" or look out for a getaway vehicle.
But I freely admit the above is speculative at best!
All IMO, etc.
Points a-g I pretty much agree with, apart from the speculation about the front door key. A plan conceived prior to the arrival of the McCanns may have included the key as a necessity.
If the apartment had been under surveillance then a potential abductor would surely have seen which door the McCanns were using. A quick check on the patio door would have shown that it was the easiest point of entry and a watcher could have observed from the alley or the garden of 5B. Exit could have been either via the same door or out the front, depending on the lock and a getaway vehicle anywhere out front.
I'm not sure about the "insider" part. I'd more readily accept that if the motive had been simple burglary and there had been signs of a disturbance inside 5A. The unseen/unheard removal of Madeleine from the apartment took a greater degree of planning/risk with an unknown motive but imo much boils down to the question "who knew?"