Even more important is that Rosenstein is not the affiant on the application. His role was analogous to that of a notary. He's got thousands of people below him and would never have time to review every application's information anyway.
See this:
I don't think they were dirty because of blackmail, but it's not out of the question to think they were in no-win situations in which there was nobody to report the criminality they were aware of. Instead, they bide their time until a righteous leader takes control.
Keep in mind, if they really were dirty themselves, then even an exoneration of Trump would be suspect. It can't be true.