>is it possible that there was such an incredible threat to national security that even an auditor could be convinced by a federal agency to give a false report?
Absolutely possible, even somewhat plausible but unlikely. Conspiracies are hard to maintain "2 can keep a secret if 1 of them is dead".
You could outright threaten someone with any number of means to get them to comply with your wishes, this is how espionage often works at a state level "ho ho, you like underage boys Mr. Smith, we have these photos of you, you will help us spy on your government!" or "You owe us much monies from your gambling, you can give us information or we make your life very difficult", simply using sex to ease someone into complying (sexpionage both as blackmail and as reward/entrapment. A possible famous case of the blackmail route was with the NSA in 1960, see: https://en.wikipedia.org/wiki/Martin_and_Mitchell_defection ), finding actual irregularities in someone's finances and threatening to go after them for it etc.
Again, possible and somewhat plausible but probably just a journalist fabricating a source or being misled by one. Yellow Journalism is a thing after all https://en.wikipedia.org/wiki/Yellow_journalism and if you look at papers in the 19th century you see all sorts of outright fabrications just to sell papers, like the Great Moon Hoax https://en.wikipedia.org/wiki/Great_Moon_Hoax
A US intelligence agency nowadays would just issue a National Security Letter and force you to not say anything otherwise you go to jail. Most people really don't want to go to jail. An intelligence agency would really only resort to blackmail or extortion if they were operating in a foreign country where they couldn't outright bribe someone.
I imagine that is defeated by simply saying "I refuse to lie", which could be career suicide but it's still an option. I can see how legally they could say "you can't reveal that information, it's a matter of national security" but I don't quite think we're at the point in America where they can go "Say this or go to prison".
It didn't stay too secret, Soviet atomic spies penetrated the program.
Emil Julius Klaus Fuchs for example was convicted of supplying information from the American, British, and Canadian Manhattan Project to the Soviet Union during and shortly after the Second World War https://www.wikiwand.com/en/Klaus_Fuchs
Absolutely possible, even somewhat plausible but unlikely. Conspiracies are hard to maintain "2 can keep a secret if 1 of them is dead".
You could outright threaten someone with any number of means to get them to comply with your wishes, this is how espionage often works at a state level "ho ho, you like underage boys Mr. Smith, we have these photos of you, you will help us spy on your government!" or "You owe us much monies from your gambling, you can give us information or we make your life very difficult", simply using sex to ease someone into complying (sexpionage both as blackmail and as reward/entrapment. A possible famous case of the blackmail route was with the NSA in 1960, see: https://en.wikipedia.org/wiki/Martin_and_Mitchell_defection ), finding actual irregularities in someone's finances and threatening to go after them for it etc.
Again, possible and somewhat plausible but probably just a journalist fabricating a source or being misled by one. Yellow Journalism is a thing after all https://en.wikipedia.org/wiki/Yellow_journalism and if you look at papers in the 19th century you see all sorts of outright fabrications just to sell papers, like the Great Moon Hoax https://en.wikipedia.org/wiki/Great_Moon_Hoax