These people are being pretty fast and loose with the word "etiquette," aren't they? They don't invite 1/2 of an established couple to the wedding and then they tell you that you didn't rsvp fast enough to be able to attend surrounding events? But they are trying to (and apparently succeeding to some degree) hold up the grooms family for funds they can't afford because of "etiquette?"
I think I'd ask to see which etiquette book they are referring to, because it's not one I've ever come across.
And I would totally RSVP yes for the luncheon/reception just to see what their reaction would be. You know that you really should not go to this disaster in the making, but for the sake of EHell, I kinda hope you do! I imagine all kinds of stories coming from this union.