Yes, overheating the engine took it out completely, the conditions you explain sounds exactly like a seized up engine. I have seen the same scenario but this engine turned over, I did not realize that it was seized until I tested the current draw of the battery for the starter, instead of about 100 Amps, it was pulling 600 Amps, then I tried to turn it with a breaker bar the same as you and I could not turn it, needed to change the entire engine on this Diesel BMW, the starter was very strong so I was caught off guard on this model. Good luck with your car.