Sufficient credible evidence isn't a standard I've ever heard of.When there is sufficient credible evidence. Before the NFL decided to announce that Sly was not credible, I think they would have a stronger argument to request the interviews. Now that the league has admitted there is no credible evidence, I don't see the basis for the interviews.
I'm not sure what your point is. Are you claiming that the players have some sort of legal right to refuse to participate in an investigation that isn't based on sufficient credible evidence? (Whatever that means.) What And who is going to make that determination? A federal court?
If the players refuse to be interviewed, they'll be suspended, and Goodell will uphold their suspension when he conducts the internal appeal. If they challenge his ruling in federal court, they will lose at every level.