I'm not Norm, but to me it's clear that an RSVP that specifies "regrets only" means that regrets are perfectly OK. Actually, I believe that anytime you see RSVP it means that it's OK to regret, because the purpose of the response that's being requested it to tell them whether or not you're coming.
It seems to me that an invitation that you're "not allowed" to decline would not say RSVP, because you're expected to be there and a reply would be superflouous.
Of course, Norm and I disagree about whether you're allowed to decline an invitation from the Captain. He says it's written in the etiquette books, and it probably is, but I think that in common practice today you're perfectly free to decline such an invitation. I think Ernie agrees with this, although I don't want to put words in his mouth. In any case, there's certainly no penalty for doing so.
Anyway, I'm quite sure that you were on solid ground in declining an RSVP invitation. And, considering the situation, it looks to me like you did the right thing!