I have a better idea for your case. Defamation cases require specific evidence showing affirmations of fact that caused damage and if speakers and posters can be positively identified the statements, if otherwise authenticated, could be offered into evidence. Retain an attorney and ask them to do one or all of these things to the case: BLUE LINK BELOW
Law Offices of Andrew D. Myers, North Andover, MA & Derry, NH provide answers for informational purposes only. Actual legal advice can only be given by an attorney licensed in your jurisdiction, thoroughly familiar with the area of the law in which your concern lies. This creates no attorney-client relationship.
Have you actually been sued? If so, it's important to contact a lawyer as the filing triggers deadlines for you to appear in court. Also, if you have liability insurance covering your business, that company should be placed on notice. But to your question, yes, any evidence that is relevant to the claim or your defense is admissible, subject to requirements of proving the evidence is authentic and that it meets other evidentiary hurdles that your lawyer can explain.
Potentially yes, but it will depend on how presented and by whom. I can't tell from your question if you are asking if you can present them or if the Plaintiff can present them against you.
One thing to keep in mind with defamation is that the Plaintiff must prove that there was a statement of fact (not opinion), that was false, that you knew or should have known was false (and there's a wacky legalistic definition of what that means), that others heard the statement, believed it, and that as a result the plaintiff was harmed.
That said, do the radio transcripts or Facebook screen shots serve to show that you had reasonable basis to believe that what you said was true? Hard to say, since they seem to be contemporary of your statement and not factors in how you formed your opinion in the first place.
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