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.
For the past 32 years ,at night after every day of diving we use several drops of "Ear Beer"
Mix 40% white vinegar + 20% glycerine + 20% alcohol and put in a squirt bottle.
Apply inside each ear. Never had an infection and we have dove in some sewer like conditions.