I think that you are making an unjustified correlation between the food you are giving this snake and the decline of its health. Of all the ridiculous things that rodents have been blamed for causing problems in Easterns, one thing that has never been mentioned is regurgitation. Instead there are many other likely causes in this case. One certainly could be heat. Perhaps chronic exposure to sub optimal temperatures resulted in the demise of the snake, and perhaps the damage was done by the time you attempted to try a different temperature.
Another possibility is parasites. Parasites can be introduced through ANY form of foods, and I would suppose that the chance of a heavy parasite load would occur in a collection with other wild caught reptiles that have never been tested/treated for parasites. The one that you gave us has been doing fine, but we also immediately put it on a prophylactic treatment of fenbendazole and metronidazole the week that we got it. I will do a follow-up fecal float in another few months. Hognoses apparently can carry a heavy parasite load, and I honestly think this is the cause of death for a lot of captive Easterns.
Another thing to consider is that not every animal is meant to survive, and congenital defects certainly can take a few months to manifest.
Since most hognoses probably get overfed, I would suggest to you that you evaluate the prey size you are giving the hognose like you had mentioned doing. I have been unable to find any information on rodents being detrimental to Eastern hognoses, and certainly the prey item or prey size wouldn't have been a direct cause of death in this case. We could speculate all day, but in the future I would suggest getting a gross necropsy performed, as that is the only way to conclusively evaluate your husbandry. A heavy parasite load and a systemic infection would be evident even in a small snake.
-----
"I know the human being and fish can coexist peacefully."
Governor George W. Bush, Jr.
"Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us."
Calvin and Hobbes (Scientific Progress Goes 'Boink', 1991)