Well...too late to help this pup and maybe the entire litter, but I'll try. For starters, I am a vet, and I find that people who "don't trust vets" are often the ones that are reluctant to spend money then ****** when the vet can't wave their hands over the pet and heal it for free. With that out of the way, seizing in a young puppy can be hypoglycemia (low blood sugar) temperature problems (remember pups can't maintain their own temp for weeks, and people often overheat them) exposure of the mother to certain infectious diseases (distemper, toxoplasma, etc), as well as other environmental toxins, maternal septicemia, etc. The tough part is there's no way for you or anyone online to tell. It sounds like you are doing a kneejerk reaction of I don't want to talk about this and I'll never have another litter, but you still have most of this litter (for now at least) and if you don't want to watch the entire litter die, you are going to have to return to the vet, opt to pay for testing, possibly take the dead puppies to the state lab for post mortem where they can try and determine what happened. It won't be cheap but breeding dogs the right way is never cheap and never a money earner.