As a last test, try disconnecting the cable T-lug and try to rotate the arm contraption.Ok so I loosened it up, and did another videos, still no movement. I can pull the cable back and forth when it's loosened up so I don't think it's siezed. I tried moving that arm contraption and I couldn't get it to budge with my hands, I tried both sides results are the same for both. Could those arms be seized up?
It should rotate back and firth freely. If not, your calipers are shot.
Here's what I did to reinforce my return spring.
I think that's what you call Jerry Rigged. Lol