Not if "moderate" means that every user comment has to pass in front of a lawyer, or at least some paid moderator, before it's approved and posted.

Obviously this is all speculation and depends on the wording of the law.

Sure. "Small" being of course up until, say, 100 posts a day? So you just need to hire a guy that is current on all legislation and court cases related to copyright in all states so he can determine fair use.

That's probably not a problem since you'd provide him with a comprehensive database containing all copyrighted material ( audio, text, visual, computer code) and an algorithm that detects legal equivalents of these materials in your forum in real time.