I have the military discount and my PVP told me the same...only a Carnival employee discount trumps the military discount.
I went on a Carnival Employee discount from my daughter once. It was a good discount but I could not choose a cabin or even category. It was what they had available. Turned out a cat 4a on deck 1. I loved the cruise and the price was great but never again, I want my balcony and will pay for it. As far as the Military Discount, once you have sent your DD214 in, it will be on file and you will be eligiable after that for the discount without extra hassel. It was not bad once I found my DD214, just faxed it in with a copy of photo passport and I was done.