I will try hard to not play doctor here because that is not something I can do. I have never read about fats turning to sugar so I don't know that there is any problem with salmon. I have not worried about fat content of a meal but it is good that your cousin has lost weight. I have many years experience with developmentally challenged students and employees living with us, so I know it is sometimes difficult to keep the inspiration going for good eating. Salmon is leaner than beef and pork so it is an excellent choice as ir provides omega-three benefits which are important for good health. 6;1 is a good number for A1C but unless you are disciplined, it may creep up over time without you noticing it, as mine did until I worked harder on my diet. At this point, skip the expensive apparatus and test in the morning so you know what to cook, or test at night to see how much you have overdone on the carbs. I have the Mayo diet book and understand that they also have a diabetic book which I think would be a good reference to follow.
I don't know your cousin, but your biggest issue, from my experience is he will tend to develop bad eating habits for comfort food and can easily sneak behind everyone's back to make it happen. Most handicapped people I have worked with or taught at school smoked. It is a stress thing and everyone knows that eating comfort food can ease stress for a short while. And then you eat some more.
The number I learned is that anything over 126 is diabetes and under that is pre-diabetes. Even then, it is a number most diabetics would love to reach and many do not. It is the really high numbers that do damage over time,, as I witnessed with my late husband.
It has been my experience with elderly people I know that they think diabetes is a funny joke and a result of doing what you want to do, so they think they are entitled to eat desserts because they worked hard all their life and now can enjoy eating without damage. I left a church full of elderly who focused on fat foods and desserts as a reward in their life, and they didn't appreciate my discipline or respect my attitude toward good health. So diabetes was not a social concern for them and I was the odd person out of the social circle.
I understand about your epilepsy because it is not a disease of choice, but diabetes can be a choice it you do not eat well. My number one issue is that people eat balanced meals and snacks, with protein, veggies, fruit, dairy, and whole grains for good health. The Mayo Clinic diet book does an excellent job of explaining in practical terms how to eat.
Remember to reward your cousin with anything but food. He always needs praise for doing well. And he is doing well right now.
Checking the blood sugar guides you on how to cook. Your doctor may not have confidence that you will do the discipline with daily testing and he prefers a machine which records the body function. It has done wonders for me to become knowledgeable about keeping my body healthy since I have so much garden produce to use. The test strips are outrageously priced these days but Medicare pays for mine now. In the beginning I paid for them myself because it is so helpful in knowing how to eat. I don't need to test to confirm that I have eaten the wrong foods or eaten too much at a meal. And I never eat a sweet snack alone with nothing else at 4:00 p.m. No No!
I consider myself at well person, not a sick person, and I use food wisely to extend my life. It is a little harder with your cousin but I admire your respect for him to do the right thing with food to extend his life. Mozart died before the age of 40 from diabetes. He was ignorant of how to make the choices for a longer life. But he was a brilliant musician and wealthy so he indulged badly. Think about it.
Dorisena

Jake: I thought I'd just answer a part of your post I didn’t see addressed; of course it’s kind of difficult to follow the posts, so I may have missed it.

You asked: “ I saw “no sugar ice cream” and sugar free candy. Could he have either of those?”

What matters when you have diabetes is the carbs, which is what will send blood sugar high. I myself keep to around 40-45 carbs per meal. If you look at most labels, the low sugar or no sugar version of candy, ice cream, etc often has higher carbs than the sugar version.

Many of these snacks have over 20 carbs per serving or higher, so would you rather have a bowl of ice cream and not eat a meal? That’s what I thought.

It doesn’t mean all snacks are off limits. I ofter have a hard candy like Werthers, Brachs, etc before exercising to give me a little energy boost, as most have only about 4 to 5 carbs per piece. And there are a few low-carb ice cream types; Breyers Carb-Smart chocolate covered ice cream bars have 11 g carbs minus 3 g fiber (8 net carbs) which is a nice snack.

Learn to read the label before you buy, take into account how many servings per pack, carbs per servings, etc. Often the numbers highlighted on the front are come-ons, as a normal piece would be counted as multiple servings.

Hope that helps.
Howard