If your being stationed overseas was for military service, then first of all, thank you for your contribution. If it was for a civilian job overseas, I can see that it put a great strain on your family as well, so that's very difficult.
It's understandable that your wife wanted/needed help from family while you were away, and it's customary for grandparents and aunts and uncles to be less strong on the discipline, however frustrating that is for the parents. I think it's very understandable for you to want to take more of a role in the childrearing since you've been home, but you have to do this "re-entry" more gradually, lest your child resent you. No parent should turn food choices into a battle because the child a) has the control and b) will resent it, but it's worse if you make a sudden and abrupt change after not being in the picture for a while. The child will make this about YOU being the problem, and you don't want that.
I think calling your wife's family members names is going to let such hostility build up, it's going to be a problem for you. And your child, for better or for worse, was used to these 2 caregivers while you were gone. I think it's better to say "It's time for Daddy to have a chance to integrate into the family we were lacking for so long while he was away" rather than "you're idiots and control freaks." I gather that the therapist thinks that more consistent structure will help with the ADHD, so you (or better, you wife) can say that as well: "Shuttling our child back and forth to different houses makes this disability so much harder to manage, and while we appreciate all that you did for us while D. was away, it's time to focus on a more consistent routine in our own home."
They should not be imposing their routines on your home, and so they should only visit when invited. They need to take a step back from the parenting - which, by the way, your wife may be have been too careless about delegating to them. So, while family is important and inviting them over is nice, it has to be by invitation only. Your wife should handle this. If you were in the military, it's possible that you were used to giving or taking orders (or both) and so you think that works in civilian life. It doesn't. I understand your desire to assert your parental rights here, but it's going to backfire with your relatives as well as your child, and your marriage will suffer.
If there's screaming and hollering (such as by the other aunt), that's obviously negative behavior, and I hope your wife is the one who said, "I'm sorry, but we're not going to allow our child to be around that kind of talk." But again, if it came off as "D. doesn't like it so you're out" (even if that's not what she meant) or if you were the one to say,"You're out of our lives forever," it's the reason for this stress and strain you are feeling now.
Now, there are ways to handle a picky eater. Some of it involves making meal time (and meal preparation) fun, family affairs. Not "you have to eat this because it's nutritious" or "you have to stop eating junk," but more "this is fun family time and we're all eating great, fresh foods! Yum!" You can also learn to make super healthy chicken nuggets (I use wheat germ, bran and crushed almonds as the breading and frying in a heart-healthy oil like coconut or avocado or olive). You can make super healthy pizza by putting chopped spinach in the sauce and piling on veggies. You can expand to other things too: a child who likes getting take-out Chinese food can be gradually shifted to homemade stir fry meals with all kinds of things (we do amazing stir fry with tofu and chicken and a dozen different veggies with Chinese style brown rice). A child who likes Mexican food can be easily nudged gently toward a taco bar or a nutritious veggie & bean filled enchilada made at home. A kid who likes pasta can be shifted towards a nutrient-rich lasagna or stuffed shell meal with all kinds of hidden nutrition (I use tofu and cottage cheese with veggies in the sauce). So there doesn't have to be a ton of drama and control exerted all the time - parents can be team leaders and coaches rather than drill sergeants.
I'd suggest you and your wife work more closely with the therapist on what changes you can make in your daily interactions. That means making some stylistic and vocabulary changes to show more patience and cooperation. Your anger at the situation has to be something your child is sensing, and it's going to make everything more difficult. I'd also look into a cooking class meant for parents of kids like yours - check the Y for classes, and talk to the children's librarian about some of the great cookbooks with mouth-watering photos designed to make meals appealing to kids.