I don't do well with most of the "diabetic safe" desserts at Disney. The sweeteners wreak havoc on my digestive system! And just estimating carbs I don't think that they are much better for me than the regular desserts. We used to do the old dining plan which included an appetizer, entree and dessert. I would just not eat the dessert and let DD and DH split mine. But even that was way too much food for me. When they took away the appetizer, it really just became a less economical choice that buying meals for us.
Unless you are doing
free dining you might want to do the math and see if you might be better off to abandon the dining plan. If you and DH are both diabetic, maybe you could each order an appetizer and split an entree. I can tell you that DD and DH have both split with me with absolutely no problem. I've also ordered an appetizer as my entree with no difficulty. Or asked that things be prepared without sauces, etc.
Unfortunately Disney does not give out carb counts for their restaurants so you are estimating. I do find that at Disney I tend to go low. I cut back on my insulin usually but keep up with the Byetta and meftormin. I test more frequently and carry glucose tabs with me - when I go low I get very cranky and DD and DH just want to get my glucose levels up quickly! If I realize I am going low before it is too late, a Mickey Bar is a great treat!
The last time I asked about substitutions on the dining plan due to special dietary needs, I was told that only the standard accommodations would apply - i.e., the chef would be willing to speak with you about ingredients and/or prepare you a special dish. Swapping desserts for appetizers or side was not allowed. An individual CM may give you a different answer so you have nothing to lose by asking!
The healthiest thing I could think of at a counter service would be the apples they substitute for french fries on request - maybe they would be willing to give you those in place of dessert. Or they have grapes for the kids meals, as well as carrots.
Not on the dining plan, at the counter service places I usually just order the kids meal. It is plenty of food for me.
Have a great trip!