IMO access via a key is entirely plausible, although it is one of a small number of plausible theories.
In this scenario the window may be a distraction; perhaps opening the window/shutters was intended to draw attention away from the use of a key to gain access, or to give the impression that Madeleine had climbed out the window and wandered off, in order to buy time/confuse.
But I believe, from the evidence in the files, that it is "almost certain" (to use SY terminology) that MBM disappeared from the apartment after the MO check around 9:30 (providing parental involvement is ruled out).
This would only require minor modifications to Sadie's theory, in that JT could have potentially seen an abductor when she went change places with ROB after her main meal.
It is a pity the statement from Crecheman is not available, which may help refine what JT may or may not have seen.
All IMO.
Sadie had formulated much of her theory before we ever heard about Crecheman being traced by SY; I think part of the pity in that episode is that more effort hadn't been put into doing that by investigators closer to the time of Madeleine's disappearance.
I have never been totally convinced by DCI Redwood's eureka moment on that one.
But I don't think crecheman's appearance changes much if anything as far as Sadie's theory of what might have happened is concerned.
I agree that the mystery of the window was never about in or egress it was about something else entirely and that may very well have been to cover up the possession of the key.
No other such 'clue' was left in any of the Ocean Club burglaries and with the exception of Mrs Fenn's disturbing experience, nothing to indicate the MO used.
Sadie has suggested that the window had been opened either as an escape route or to pass Madeleine through. As well as expressing the opinion in the instance of 5A there might have been a requirement to deflect attention from something else concerning the crime ... use of a key for the door?
I doubt if even whoever raised the shutter and opened the window in whichever order that was done could ever have anticipated the way in which that action was destined to influence the direction the investigation would take.
The newspaper headlines the day after Madeleine vanished about discrepancies in statements and badly told stories about staged crime scenes must have combined manna from heaven with a get out of gaol free card. In my opinion had they planned it that way, it couldn't have worked out better for them.