Broadly, publishing a false statement of fact about an identifiable person that harms their reputation, with some fault by the publisher; details vary by state.
Not necessarily. An opinion that implies undisclosed false facts, such as suggesting a named person is guilty, can still create risk.
Repeating a defamatory claim can still create liability, so treat sources' accusations as claims to verify rather than content to republish.
Usually not. Most witnesses, relatives and people questioned are private individuals, who generally receive greater protection.
Yes. Giving a fair opportunity to respond to serious allegations, and reporting the response, is good practice and a useful safeguard.