I suspect that the parents either knew or suspected that Bobby drowned in the lake and had gone with the kidnapping story from the start, then had to double down when the police presented them with some kids.

Yes, but a great white was found with human remains in its stomach, confirmed by a group of scientists. Also, the attacks ended after it was caught.

It remains a mystery; perhaps it was a coincidence of a bull and great white shark at the same time and place. Some have hypothesized these fairly uncommon sharks for the region were driven north due to the Spanish-American War.

All of this is summarized from my reading of the Wikipedia page.