I get a compression load in excess of 35,000lbs/ćin and that assumes the circumfrence taking an equal load which we know it doesn`t. A simple safety factor of 2x because the off axial nature transfers the compression to one side of a circle would then give 70,000lb or about four times more than you were expecting.
We always wish for a static load but just don`t get it in the live load world. From first hand experience I know the 6" sch40 steel pipe even filled with rebar and concrete will be unsatisfactory. A friend used a 10` piece 4` into the 1yd concrete footing. He is an architechtural engineer. The 6" refractor would yield to gentle breezes and wobble when one adjusted focus, align the finder, etc. Massive and strong is not always enough. His remedy was to bias the load.
I am not sure about the 1.7 cycle vibration rate, sounds a bit low unless that includes the 850lb load. Either way the fact that it is sub-audible does not mean it is neglible. Our California Earthquake are sub-audible but the din from crashing objects can be quite loud.
I think your discussions are good though as it points to the fact that one should include the various elements. Only one thing I might differ on and that is I would buy rebar bolts or J-Bolts. Rebar is a very soft metal and pipe threads are meant to seal and are not strong. It would be useful to see your calculations. Very interesting.
Cheers and Clear Skies,