No it won't shear the splines off if park was hit while moving. . . For the record, I base my words on 40 years of being in and around the car business / all sorts of machinery and seeing the results of someone hitting park at a walking speed.

As for my previous thought that a spacer was missing, I adjusted my screen and can see the governor drive gear is intact so the stack up was likely proper.

This is the result of long term wear called fretting, this is where two metal surfaces are slowly rubbing against each other causing them to wear. The metal generated during fretting is a fine powder that, given the location on the transmission, slowly made it's way into the fluid. When fretting occurs in a dry system, you will see fine brown rust around the joint.

I see no evidence of slivers of the alleged single catastrophic failure that resulted in sheared splines, if the drive flange splines had failed at the root, there will be small loose triangular pieces of metal. The trans shaft still has somewhat of a spline profile and definitely has not failed at the root. All of this points towards a long term failure, or even a the drive flange wearing out in the past, a replacement installed on a worn trans shaft and the wear process continuing.

And. . . .why are the parking gear splines / trans shaft in that area still intact? Why are the trans splines still perfectly straight? If this was an over torque, the entire trans shaft will be twisted to some extent.

It is well documented on this list that drive flanges wear out the internal splines resulting in sudden loss of drive. Any bang noise could have come from the drive line suddenly becoming unloaded.