If Gerry's alibi is solid then I can't believe the Smith family saw Maddie. What kind of planned abduction involves carrying a child across a town in full view of the world? The only way I see it being Maddie is if she wandered out of the apartment and was grabbed by chance.
No planned abduction would imply carrying a child through the streets of a touristic village where the chances to meet someone were high. IF, and I stress IF, the child being carried by that man was Madeleine, it could only be explained by an act of desperation.
I understand that the first responders (GNR police) thought the child had walked out of the apartment, but if that had been the case, despite the family had taken almost an hour to call the authorities, the little child would have been found much earlier and the alarm would have been given. After all Praia da Luz was not the Sahara desert, there were people there, plenty of people.