There are no laws against "interfering" as you call it. Quite the contrary, you're free to voice your objections to either party in any contract at any time. If you have some factual information to divulge in the process, all the better. The only danger is if you invent something, at which point it becomes slander.
Sorry to disagree with you on that. There is a long and well-established line of cases in all of the states that precludes you from doing it. It's called "intentional interference with a contract." Do a Google on it, and you'll see.
But you are right that if all they did was exercise their freedom of speech, then that makes it harder to sue. Not impossible, though.