The rule is any discounted rate will not be eligible. It used to be hit and miss but with the new method, seems to be 100% denied on any casino related offers.
Upsell offers are on your cabin location and category and Carnival's need to sell underperforming rooms. It does not matter what offer you booked under. The upsell department doesn't know nor care.
In Miami, passport holders and BC go in two separate lines. Passport holders can go in the Birth certificate line but birth certificate holders can not go into the Passport line.
Yes, the amount you prepaid will show as a credit. I guess it is because people add on to their services onboard so you are basically buying a "spa credit".