Most certainly not and he has paid a hefty price because of the malicious insinuations made against him. He wouldn't have offered his services for free the very same morning if he had been involved.
No-one made malicious insinuations IMO. Some people - and not just the three from the group, genuinely thought they saw him there.
IMO it was either a case of mistaken identity or when he realised that the PJ were gunning for him he panicked and said he wasn't there at all rather than AFTER he heard the sirens going off.
I don't think Murat had anything to do with Madeleine's abduction and I believe his mother also knew that it was not possible for him to have had a part in it because he was with her up until they heard the sirens.
This is pure speculation on my part - but I can imagine that if a mother knew for definite - by the evidence of her own eyes, that her son was with her at the time the abduction took place and she also knew all about the PJ 'robust' interviewing techniques, and her son told her that he had got scared and told the police he was in the house all night, then she might just agree to back him up on that. From her POV she would not be interfering with the case in fact she would be removing a person whom she knew to be innocent from the equation.
It's clear that Mrs Murat did know all about the fear of locals to contact the police because IIRC that's why she set up a table for them to anonymously leave any information.
Those from the group who were sure they saw him on the night of the 3rd did not back down when they had a face to face meeting with him over this. That means they were prepared to look him in the eye and say 'I saw you there'.