I don't pretend to know how Scotland Yard operate
... but here's the thing
How can they possibily say that the McCanns were not involved in their child's disappearance unless there is actual, conclusive, and definitive evidence to the contrary ?
There is not is there ? ... otherwise they'd say so, wouldn't they ?
So the most the McCanns can expect, or hope for, at this point, is for Scotland Yard to say they are "almost certainly not involved
Which raises another conundrum
Scotland Yard said that the man Jane Tanner saw was 'almost certainly' not an abductor , didn't they ? .... and the McCanns felt that 'almost' wasn't good enough ... didn't they ?
If an 'almost' certainty is not good enough for the McCanns where Scotland Yard is concerned, why should it be for us ?
As I have put forward in the past regarding this point, I think that Scotland Yard are being deliberately vague in the language they are using.
They have said that they do not claim to be able to solve this crime, and in my view, for what it's worth, their language in all the interviews they give is in line with that.
Most terms they use, such as 'drawing everything back to zero', are deliberately undefined. Statements like 'Madeleine may be alive, or sadly she may be dead' can seem facile and contradictory - and they are deliberately so. Likewise, they say they are 'almost certainly' sure they have ruled Tannerman out. What does that mean? Vagueness and ambiguity seem to be important here.
If Scotland Yard fail to get to the bottom of what happened to Madeleine, then at the very least they will not have egg on their faces in having posited a specific theory which they could not prove.
As for the McCanns themselves, Scotland Yard have said that they are not 'suspects', and I disagree with you here because I think Scotland Yard
must have taken steps to satisfy themselves of the veracity of that before saying it.
Nonetheless, there remains the possibility, according to what we know or don't know so far, that information may come to light at some point in the future which would implicate the McCanns, and in that case the language of Scotland Yard would again be important. DCI Redwood uses the word 'suspect' - a status that can be imposed or lifted according to changing information and context.