Careful monitoring won't save a snake either. My sister-in-law used to work at a pet store, and the BPs there ate live food. In went the rat with a young snake, and the rat got in a chomp because the ball missed and bit off a portion of the snake's skull. Unsurprisingly, it had to be euthanised. Oddly enough, she was RIGHT THERE. She hated feeding live to the animals for that reason, but the store owner wouldn't listen until he finally lost a snake.
A large mouse could easily do the same to an adult corn snake. If your snake won't take a prekilled, either try it on other snakes or freeze it and try again later.
Also, the CO2 used to humanely kill rodents is NOT in the blood of the rodents. Our lungs are not designed to absorb CO2. Instead, it mostly sits there and takes up space that oxygen could be using, resulting in a normal build up of CO2 in the animal from cellular respiration due to a lack of incoming oxygen. This results very quickly in unconsciousness and death, particularly in small animals.
That said, the stomach nor the intestines are designed to absorb carbon dioxide so elevated levels in the blood of the prey item would not matter at all.