580c bearing carrier has uneven brake pad surface - how to remove phillips head screw?

Redoing brakes on my 580c. The bearing carrier has uneven grooves on the braking surface from someone running it metal to metal. I assume I need to remove the bearing carrier and have it machined flat.

I've tried removing the Phillips head screw, but it won't budge. I've tried a regular Phillips large screwdriver and I've tried a large #3 bit with an adjustable wrench on it. I sprayed the nut with PB Blaster, but doubt that will help since it can't get in there very well. I also tried...

580c bearing carrier has uneven brake pad surface - how to remove phillips head screw?

580c bearing carrier has uneven brake pad surface - how to remove phillips head screw?