The are two areas of responsibility and accountability.
The first the McCanns were able to circumvent eloquently 'she's been taken' and in the light of day after a few consultations 'within the bounds of responsible parenting' Ask yourself, if you were on the razz and left the kids home alone, would you want to be singing within the bounds of responsible parenting. Thus this all falls into the category, we didn't do it, it was the abductor
The second something happened in the apartment, whatever that something was. Whatever it was, even an accident that might have been preventable or responded to had the parents been present, leads to only one area of responsibility the parents. After all they should be good first aiders. So depending on the response to the situation the 'first' could become and excuse for the latter.
Going with the flow of the first - the web is then weaved of the responsible checking, the famous timeline. Six years+ of it - all happened whilst Mr McCann checked, chatting with JW whilst JT walked by and saw the abduction in progress. REALLY. Really!! No - whoooooooooooooshed it never happened that way. She merely saw a parent with child returning from the sleep in creche. But it filled the gap for six years, even though the parent is apparently walking in the wrong direction, this must be the best piece of police work FULLY UNEXPLAINED by the MET, believe it, 'coz we say so!
So let off the hook of what must have been the blind, deaf and dumb check by MO for six years, the fall-guy. The alleged abduction happened after his check and JUST before Mrs McCanns. If indeed Madeleine was in the arms of a man sighted minutes later by the Smiths. Who we have all known about within weeks of Madeleine's disappearance.
Anything is possible.