Everything sounds ok to me. Hot spot could have been a little low at some point, but not enough to cause any huge problems. Just curious, was the rat frozen thawed or live? It's possible that a thawed rat may not have been completely ready to be fed and could still have a cold spot inside, but a ball python probably wouldn't eat in that case. Very strange.
If I were you, I would put the body in the fridge (I have read not to use the freezer) until you can get a necropsy done. The necropsy will tell you if it was an accident or something you may have done. It's a good idea to do this because you have other snakes. If it was a disease of some sort or a husbandry error that you didn't notice, then you'll want to know so you can be sure your other snakes are safe. Sorry for your loss.