You can, but be prepared to eat a lot of chicken ceasar salads without croutons!

I'd try to bring in as many snacks as you can. Maybe go to their website and read up on what you can and can't bring in with you. For a splurge, the Moroccan restaurant for dinner in EPCOT is semi-beachy with grilled or stewed meats over couscous and is kind of a fun night anyway. Just be sure to make a reservation for that morning at an early enough time so that you are finished in enough time to go back outside and stake out a spot for the nightly fireworks! You might even try sending them an email off a contact point on their website and say you are "pre-diabetic" and have special dietary requirements and see what they suggest. Most of their larger concession/lunch spots usually have some kind of salad and diet soda or plain water.
????: South Beach Diet Forums http://www.southbeachdietbulletinboard.com/south-beach-diet-forums-phase-1/5002-disneyland.html
Just do the best you can knowing what you know about what constitutes healthy eating now. You'll be walking a lot too, so that helps!

And just enjoy the experience. Most of their food is kinda crappy anyway, so you aren't going there for the food experiences to begin with.