I'm on my third housing, second Subal. I have always attached about 2-3 sq inches of zinc to the housing and have never encountered any problems with corrosion or tight screws after a 16 day dive trip.
Position the housing with solid support under the problem knob. Rotate knob with set screw to top (up) position and apply several drops of WD40 (or maybe the PB 2000) to set screw. Let soak about an hour. With your assistant helper holding the housing/knob, insert the Allen wrench and apply ccw torque and soundly tap on the top of the wrench. Some times the impact will break loose the set screw.