You might be able to talk to the owner driving the vehicle when the pan hit something. He may be friendly enough to tell you the following sequence of events.

For example was the car shut down or driven. If driven until it tightened up or overheated seriously. What he states may or may not be important.

With no oil but with coolant in the system one may be surprised how long they can run sometimes. Usually tightening up and damage is worse after coolant is expelled. I would think the back bumper just might be consistant with the coolant having been blown out and temperatures perhaps reaching loss of ring temper perhaps. On the other hand it started up pretty easy for you.