You appear very knowledgeable and I am not able to counter any point except that the bolt was slammed into place and the rifle ready to fire.
It just didn’t wait for me to pull the trigger.
I think that the bolt did close alright but didn’t completely lock (rotate into position) and firing pin struck the primer with its inertia. It should not have been able to do that - because of that slot in the lower rear of the receiver. I’d also guess that it was relatively new rifle and not heavily lubricated, which added some friction to the bolt closing process. That caused the bolt to slow a bit as it closed, the rotation wasn’t complete but the firing pin continued forward unimpeded and struck the primer.
Your pictures show that there was huge gas release at the breech and that gas split the stock in several places and threw the operating rod aside. The bolt locking lugs appear to be intact, which indicates that they didn’t fail - so the rifled fired with the bolt unlocked.
If I am right, the cartridge case was ruptured at the rear portion and most likely the head of the cartridge was completely severed.
I am not infallible but I have conducted a lot of investigations of weapon and ammunition failures during my career. I was also one of the last Marines issued an M-1 (1965 - carried in ITR before deplying to Vietnam).
Hope you have healed completely.
Semper Fi,
Chainmail