Well, I finally fixed it this morning!
After going back and forth with Apollo valves and where to get the replacement part they took pity on me and shipped me one but as it was on backorder I didn't receive it until last Friday. I used a "
Superior Tool 03840 Adjustable Combination Wrench" for $14 to undo the cap. The tool started to give before the cap would unscrew. The tool has a bolt and a win nut to keep the jaws from moving, I replaced theirs with a bolt and nut so I could apply more force and was then able to remove the cap.
It was apparent for a 2 year old valve that it had been leaking since installation as the pressure spring and cap were severely rusted. The replacement kit didn't include new ones so I cleaned the originals as best I could with a wire brush. Levered the old cylinder out, applied so va-sp-10 to the O-ring on the new cylinder, cleaned the threads and seating area on the valve body, inserted the new cylinder and screwed the old cap and spring on. Then turned the water back on, adjusted the pressure screw to get 50 psi and finally had a leak free valve!
Thanks for your help!