I'm very sorry that your first experience as a snake owner ended in such a way. I'm really happy that you've gotten it figured out with the store and they were able to replace your animal for a happy, healthy one!
If the snake had overheated and had sustained such intense neurological problems in that way, wouldn't the belly have been burned? I've never heard of a snake getting overheated so badly without sustaining at least some physical damage. Is it possible that the snake was inbred? Or simply born sick? Directed at anyone, just kind of musing...