Buffet opens at 6:00, MDR at 7:00. Excursions don't start until approximately 30 minutes after you are allowed off the ship, so there's no reason to forego breakfast. (room service is basically continental...no hot food.)
Crusty's times are pretty good estimates. However, the Lido buffet and MDR times do vary from ship to ship and from day to day. The only sure way is to look at your Fun Times the night before your excursion and see what the plan is.
I have also observed that the first half hour that the Lido buffet is open is just continental breakfast, no different than room service, but with worse coffee.
I am a Cruisaholic; "I'm Not Dead Yet"
Carnival Inspiration, January 17, 2014 Long Beach, CA *** Carnival Liberty, March 22, 2014, Miami, Florida *** Carnival Miracle, September 10, 2014, Seattle, WA *** Carnival Freedom, March 21, 2015, Fort Lauderdale, FL