Well, 120c is 248f, and that's definitely hot enough to do some damage in an older design motor. The
oil out the breather is most likely because the overheated motor thinned the crap out of the
oil (part of
oil's job is engine cooling). The thinner oil is much easier to work it's way out a breather. But also, thinned out hot oil doesn't lubricate as well, and could have caused some good galling inside the motor.
Did YOU shut the car off and coast to a stop, or did the CAR shut itself off and then you coasted to a stop? It's not odd that the motor seized, but it is odd if you shut the car off with it running, and then it siezed just sitting there.
One thing I can say, is that if you tried to pop start it, and it actually locked the drivetrain up, that's a major siezure on the motor. A seized ring or bearing isn't going to hold enough to lock the whole drivetrain up. There's gotta be something else going on here.
But I agree with the others, first thing you should do is call Keith.