Hey Nanny B,
I just noticed that you're a Spokie. I too am from Spokane

I don't have experiences in Cozumel or Cancun, but I wanted to let you know that you should be able to bring Lara bars, cereal, nuts, and other dry goods into Mexico. You can't bring fresh fruits or vegetables, but you can bring them on the plane and leave any leftovers on the plane when you get off. I always do that and so don't have to declare any fresh fruits. Remember to pack some snacks for the return trip too because I forgot last time and almost starved!
I don't have any experiences with all inclusive places or American owned restaurants, but I can say that you should be able to get lots of fresh fruit and vegetables. Veggies that are peeled or need to be peeled are generally safe, as far as I've experienced. If you haven't tried fresh guava before, now is your chance! The seeds are a little weird at first, but the fruit is so good. I'm going to central Mexico on Thursday and my boyfriend is meeting me at the airport with a bottle of water and fresh guava.
Good luck and have fun!