Similar issue has happened to us. The "Price Protection" guarantees you pay the lowest rate, nothing more. We had a rate of "X" and a $50 shipboard credit from a previous price reduction. We later found another rate reduction of $100. Carnival took back the $50 credit and reduced the rate by the other $50. A shipboard credit is no different than a price reduction IMHO. It's money back in your pocket whether you get a discount off your S&S and the end of the week, or it's discounted right off your rate. 6 of one, 1/2 dozen of the other. You can't "Double Dip" the discounts.
Exotic Western Caribbean - Dream (B2B) - November, 2013 (Our Honeymoon)
Eastern Caribbean - Dream (B2B) - November, 2013 (Our Wedding)
Mexician Riviera - Spirit - November, 2011
Western Caribbean - Freedom - January, 2011
Southern Caribbean - Miracle - November, 2010
Western Caribbean - Liberty - November, 2008
Eastern Caribbean - Valor - October, 2006