What would cause this brake pad wear, and any guesses about some unusual symptoms?

I'm doing a complete 4 wheel rotors+pads replacement on our 2011+ 250k mile prius.

Rear wheels now complete (rotors were at 7.8mm.)

Physical evidence of issues:
* Right rear outside pad has worn flat on the upper (close to rotor edge) side, several mm of differential wear (See photo.)
* All four rear caliper slider pins had sludgy black grease. They moved but not easily. (Redoing with the official Toyota grease made a night and day difference!)
* Rotating wheels definitely made a small scraping sound: pads in contact at least lightly.

In the last month, I noticed:

* Occasional small vibration while braking
* The car was outside overnight in very cold weather (0-10F). In the morning, it did NOT want to roll for the first 5+ minutes until warmed up. Scary to me.

My questions:
* What could cause the differential pad wear? Could it be the sludgy slider pin grease?
* Could the grease also have caused the roll resistance, perhaps brakes still acting even though parking brake released?

Thanks for your insights!