1. How did Tom & Myrtle’s affair get covered up in the investigation?

There is never any proof of a cover up. Nick thinks that the police covered up the affair rather than probe Gatsby's life or even the Buchanan family. Tom certainly had the money to avoid intense "examination". George Wilson is a much easier target to go after. He is not sophisticated and does not have the means to prove his innocence. I think, in the end, Nick feels that the characters who populate this novel were never interested in the truth of any kind.