Am I missing somthing here.With your timing gear off a tooth or two the engine still will TURN OVER.Even with the timing chain off the motor will still turn over.He is not talking about starting his motor.Hes talking about it locking up right?A valve would hit a piston before a plug would.Unless the plug was way way to long.
Here a idea put all the plugs in.Put a breaker bar and socket on the crank bolt and see if you can turn the motor over bay hand .This may eliminate a few things it may be.
The hole thing sounds odd to me.I still may bet on a starter bind.but who knows.