I can relate to your frustrations. I recently helped get this done for my daughter. The Carnival site is not as user friendly as RCI's in my opinion.
I did not realize it until we took her to the airport, we were supposed to print out something, that indicated we had filled out all the info. online, so you might want to make sure you do that, to increase the chances you can board the ship with limited delays.
On their website, if by chance you are booking excursions, and sharing with others, be careful, because it is easy to place an "x" in the boxes, and delete the excursions you already booked. That happened to us, when a roommate was trying to book her excursions on her own credit card, and she accidently deleted two roommates' excursions. Luckily, they were able to sign up as soon as they boarded the ship. Have a great cruise, and good luck with the rest of your registrations, etc. online!