Consider Rustoleum self-etching primer before you paint with your chosen finish coat. Cave usually did not primer and his paint peels off.
Think hard before you remove the large "altitude bolt" if the polar axis angle matches you latitude. If the angle is correct no need to remove it. You can paint with it in place.
Secondary holder is a "backplate" as you call it and a shroud. There should be 3 small screws at 120 degrees at the base of the shroud which screw into the backplate. Your pic shows one of the screw holes. Remove screws and then pull the backplate away from the shroud. It may be stuck from age but it should yield.
Thanks, Kent. Yes, I figured those adjusted the tension on the inner ring, and I loosened them to see if I could get it to rotate any easier. I will certainly take your advice and not remove the inner ring. I will use the primer before going any further, thank you. The altitude bolt looks pretty rusty, but it looks fairly close to my latitude. I'm thinking it will be a bit off because of where the person lived, so I may have to adjust it. I was originally thinking from some of the other threads that I would need to completely disassemble everything, but I think not.