My first thought when I saw the droppings was also cancer. Did the vet do any labs to ID any of the possible viral causes? The white in droppings is the urates, or urine, so dehydration could cause it to be absent. Or kidneys not functioning. In this case a necropsy would really be the best option to ID actual cause of death and with labs to look at bacterial and viral possibilities. Often cancer is fairly obvious on necropsy.