I would say you RSVP yes, no mention of possibly being too sick so far in the distance. I think that unplanned catastrophic illness is an accepted excuse for bowing out later, there is no need for pre-approval on that.
If you have no other diet restrictions other than "no buffets," I would have DH contact the cousin. "Hey, we are so excited to come to your upcoming wedding. We have kind of an odd question...because of immunity issues, my wife has been told by her doctor that she cannot eat from buffets. This won't keep us from coming either way, but can you let me know if the wedding meal is plated or buffet? If it is a buffet, we just need to know in advance so that we can bring an alternative meal for her. I am sorry to request such a random detail from you so far in advance, and if you haven't nailed that down yet, I can circle back with you closer to the date. Again, we can't wait to see you, and congratulations on your upcoming wedding!"