I'm not quite sure that I follow your reasoning here ... Jane Tanner is a witness who saw a child being carried away from the place from which a child disappeared.
What do you think she should have done when she discovered that situation?
Jane Tanner is a witness -- a witness to a few things. First, she saw Gerry McCann at the gate of 5A. At the same time, she saw Jeremy Wilkins. And third, she saw a man who has now been identified and ruled out.
The only thing Jane Tanner should have done differently is -- when she gave her statement, to have talked ONLY about what she saw, and not about what others allegedly saw or did. Apart from that, what is there to criticise?
A lot of people have said disparaging things about Jane. For example, there are those who question how her memory became progressively better. There are those who ask questions about her conduct during the Robert Murat ID appointment and the subsequent rotatory interview exploring that. There are those who believe she wrongly identified 'George' as the man she saw to the private investigators (ruled out later). And there is the voice analysis expert who analysed her vocal replies during her interview with private detectives, and believed her not to be a credible witness.
But I'm not criticising her for any of that. Thanks for the question.