Of course there's a presumption of innocence, but only in legal terms. They're nowhere near a court (current libel trial excepted of course) at the moment so there is no innocence or guilt to be decided anyway. The questioning of their story is not an accusation of guilt, it's merely the equivalent of me (and others, of course) telling someone "I don't believe you" when told a story. The thing with the McCanns is that they've told their story in a massively public fashion so there are lots of people that are well within their rights to say, "I don't believe you" in whatever form they choose - provided, of course, that they don't then move on to libellous territory.
My certainty they're not telling the truth is more than just guesswork... but they have been caught out with lies already (a lie, after all, is a deliberate falsification of the facts) and this has happened on many occasions some of which, I concede, could be attributed to simple mistakes under pressure or faulty memory but many of which cannot. I'm not going to go trawling through the archives again to exemplify every single one as it's tedious to go around the track again on this and it's been well recorded already.
It could, of course, also be argued that your certainty (which I presume you have?) that they are telling the whole truth is also based on pure guesswork as there's no way that you could know for certain. You can presume innocence all you like, but you cannot know for sure at the present time.
It is precisely because they are so far from any court hearing/trial that presumption of innocence matters. Whether the McCanns have told their story far and wide is irrelevant. They are entitled to that presumption.
There is a massive difference between someone saying "I don't believe you!" or asking open questions about the case and someone, who by dint of his previously (till he was unceremoniously removed for his gross misconduct) been the lead detective on the case, decides to publish damming allegations about the McCanns. Amaral has in my view, moved on to libellous territory. He offers no proof of his allegations, merely speculation.
You claim to know of specific proven lies from the McCanns. There we must differ. I am aware of no deliberate falsehood having been proven by them. Till I see some, I will remain reluctant to believe that it does exist. There are many differences between witness statements for sure but such are to be expected in any case especially when statements are taken at times of extreme anxiety on the part of the witness which would have been the case with the McCanns just 12 or so hours after the disappearance of their eldest child for example.
As for your presumption that I am convinced of the truthfulness of the McCanns. I am only convinced in the sense that no-one has yet shown me any confirmed deliberate falsehood (lie) from them. Having never seen any such then I have no reason to believe such exists. Can you show me otherwise?
However, I have said frequently on this forum that I am not convinced of their innocence or guilt. I am entirely open-minded about what happened to Madeleine McCann. She may be dead or alive. There is no proof of what happened.
But I will not accuse anyone of a crime without evidence of them having been involved in that crime. That is where I take issue with a large number of anti McCanns. And there is absolutely no evidence whatsoever that shows the McCanns were involved in the death or the cover up of a death of their daughter.