Why is quickly cooling a a hot pan bad for it?
Specifically it depends on material the pan is made of. Just like how heated glass can crack once immediately expose to a source of the opposite temperature. For example, a cast iron skillet has the potential to crack when at a high temperature it is exposed to a much lower temperature. To answer your second question, the reason why reheating the pan wouldn't return it to its original shape is first, you would have melt the metals to reform it and second of all if a crack has occurred in the situation, it means the elastic limit has been reached. The elastic limit is the greatest stress something can undergo and return to its original shape. Beyond the elastic limit, it is impossible to return something to its original shape. I hope this helps!