I don't know that he can. We don't know, since the agreement itself isn't available (even her lawyer says IIRC that it had been destroyed with other old files, which sounds sloppy to the point of suspicious to me). I would think that typically the agreement would be signed by the complainant or her attorney on her behalf or both and by an officer or attorney on behalf of the NRA. Cain said he recused himself from the whole thing, so it doesn't sound as if he personally was a party to agreement.
Also, it seems to have been the NRA (or their insurer) who paid the money.
I don't know that I've ever heard of anyone being released from a confidentiality agreement, so I don't know what it would entail.
I think you’re right, but when he was asked he’s hedged. He should publically come out and call for their release. If these charges are bogus or there’s nothing to them it helps him hugely.