I also used Carnival's air program. Here's how it worked for me:
1. I found out about my flights about 50 days out. The information showed up in the Cruise Manager under View Cruise Documents.
2. & 3. My flight from LAX was booked with Delta. The day I had my booking #, I went on Delta's site, picked my seats, completed my passenger profile, signed up for email alerts, etc. I have been monitoring my flight for changes (none so far and I leave this Saturday--whew!) only via the airline's website. My flight has 1 connection both ways--but I expected that flying to Barcelona from the west coast.
4. My flight arrives the morning the ship is set to sail, so I don't have insight on the hotel part. However, since I had several hours to kill and would have luggage to deal with, I booked the Barcelona city tour with Carnival. Great way to see the city, kill some time, and not worry about my bags.
Have a great trip!