What caused inboard front disc brake pad to score the rotor?

I’d say a bad glue job for a lack of better words allowed the steel backing shim/plate on the pad to let go.

A rock or small metal shard was the most likely item that made the 1 in a million jump off the road and became stuck between the brake pad and the disc.

The caliper should be easy to compress with a C clamp. The amount it retracts while working on a driving car is a very small amount. Feeler gauge amount. Not a 1/8th or 1/16th of an inch but smaller. Mostly a pressure relieving.

I’d say that a helper would depress the brake pedal and physically show you that it’s clamping the disc. Try and shake it. Then release the brake pedal and you should see the caliper physically relax but no major pad retraction happens. You should be able to move the caliper.