Would it fire andbackfire if seized
Wouldn't turn over if seized. If it's cranking over as it normally does, and the fact you had this 3rd gear incident, I'm wondering some form of valving issue. Piston kissed a valve or timing jumped. Lock up however is generally lack of oil seizure or a rod broke, etc. The fuel pump does time out when the key is turned on but shout restart when you hit the start button.. Had it not been for the lock up, I'd be looking at the CPS as the reason for not starting up or squirting fuel in the throttle body to see if it briefly fires up. By the way, they've been known to seize from lack of oil pressure but unseize when they cool off. The CPS can be checked by unplugging it and hooking a VOM to it. With the engine cranking it should generate about 3v, depending on how fast the engine is cranking over. Pull the plugs for best test results. If good, squirt fuel into each tb and see if it starts up briefly. If it does, you have a fuel issue. If not, do a compression test. Might as well do that if you pull the plugs for the CPS test. So far, the PC has been ruled out.
Ron