I'm trying to figure out why the inboard pad kicked its shim halfway out from behind the pad (until the shim hit something and could go no further) and scored the inside face of the rotor
Are you saying the shim that should be between the piston and the pad got to a place where it could touch the rotor.?
That's a lot of movement.
I agree that the shim ( anti squeek device) lost it's grip over time, and probably walked with every pedal application a tiny amount till it got to the sweet spot and was grabbed by the rotor and folded.
It's also possible that the anti squeek wasmis aligned during installation.