I'm so sorry about your puppy.
The only way you'd know for sure what happened was if you were to have a necropsy done to see if your pup was ill prior to your purchase or if the vet screwed up and the directions he gave you, caused him to go into hypoglycemic shock. Many of symptoms you described are symptoms of low blood sugar and with the vets directions to with hold food and water, it seems as though low blood sugar causing seisures, coma and death, is a pretty good possibility.