Of course it would be kind of annoying to have hired crooked detectives but I don't know how or why their sins would make the Smith sighting any less valuable. It was not a perfume-stealing detective reporting it, it was an Irish family. If it was my missing child and someone said they saw the abductor, I would want it investigated, no matter what.
It was not them who hired them! How would you know which PIs to hire if your own child is missing.
The PIs were hired by Brian Kennedy who paid for them himself, chose them and made them do the search.
And if Irish man claimed that was Gerry he saw and Gerry knew it wasn't him who the old man saw of course he would rubbish the statement. Would you?