[kesaiserris]

Some theories
Hamsters hoard their food even when its limited and live on rations. Less food means smaller rations and it could be that the rations became insufficient to meet Darkie's needs.
Dietary changes even if positive can be dangerous in a sick and/or elderly animal. Case in point I had a cat Garfield as a child. Garfield only ate Meow Mix which is terrible and full of junk. When I was 18 I got a call from my mom saying Garfield had eaten a poisonous lizard and might die. She told me the poison had damaged some of his organs. Somehow he pulled through. Months later when I got back from Europe Garfield was still around and behaving pretty normally. Eventually though he did begin to develop issues he started to smell and he developed skin problems. I took him to the vet and there was no options except to try a diet to help restore liver function but that was risky. We introduced the new diet but he became severely constipated and when the laxatives the vet recommended did not work we had to put him down because he was too old and his condition too bad for surgery. I felt extremely guilty for his death because I changed his diet and his health deteriorated faster as a result. Also he loved Meow Mix and didn't like the new food and began to lose weight (he was morbidly obese). Hamsters have sensitive stomachs too dietary changes, too much fruit/veg, various illnesses and stress can all cause diarrhea.
If you weren't feeding Darkie I suspect you might not have offered him fresh water and/or substrate during that time. Which could mean that he was exposed to something like algae in the water or that he may have been reinfecting himself with some intestinal bug. Eating too little or not at all often indicates illness.
Another possibility is that Darkie consumed something inedible like hair or cotton fibers which created a blockage limiting his ability to eat. I know some pet stores sell an alternative to cotton which can, if consumed, cause blockages.
To answer your question starvation actually does result in increased energy/activity as seen in anorexics. Also Darkie could still have had Diabetes without being overweight some animals/people lose weight/appetite with Diabetes. It is possible he had type 1 or that he developed type 2 as a result of poor feeding practices at the pet stores but if he had Diabetes I believe he would have become sluggish.

I think it very likely he did starve to death via lack of food, illness, blockage or a combination of all three.

On to the practical stuff.
Hamsters need fresh water daily even water bottles must be cleaned. We give our hamster vitamin drops and the addition of nutrients makes it even more important to clean water bottles/dishes.
I have a 4.5 inch Syrian hamster we feed her a little more than 1 TBS of her mix daily whether or not she empties her bowl (which she usually does) and she manages her weight at about 4 1/2 ounces. I have read hamsters should weigh about an ounce per inch. Syrians are usually 4 to 6 inches. Check the ranges for dwarfs and weigh frequently. It can be hard sometimes to know if your hamster is eating because they store the food but by weighing them you can be sure. Also check the measurements for feeding a dwarf and use those (never go under the lowest measurement even if your animal is overweight). I think you really over complicated the feedings. We keep a measuring spoon in the food for convenience.
We give Stella a plain nut on Mondays or 2 plain pumpkin seeds if we are out of nuts (proportions might vary for a dwarf)
On Fridays she gets a hamster safe doggie treat
She gets veg or fruit maybe twice a week (we keep an eye on her poops and make sure her bottom is clean before offering fruit or veg as they can cause diarrhea) she only gets tiny pieces of broccoli, kale, kiwi book choy or the like and those pieces look silly small because she is silly small. Dwarfs are even smaller so you shouldn't be giving them even a whole grape (it's too big). If I had a dwarf I might skip fruit and sugary vegetables like carrots altogether because of the diabetes risk.
For between snacks we go with quinoa and oats usually, Stella loves oats

Never forget what happened to Darkie, at 37 I still think of how I might have handled Garfield's situation better. It might be that you aren't in a position to have pets or that you need to recruit outside help. A calendar or alarm might help remind you to feed/clean/play with your pet. Hamsters like routine and having a routine will make you less likely to forget. Schedule in pet care just as you would work or your own meals. You might even need a different job that allows you more time with your family because it sounds like you are working yourself into the grave.