I'm somewhere between 'Greatest. Post. Ever.' and 'Pics or it never happened.' Extremely informative and interesting.

The aluminum stock - that's a square tube, right?
The base pieces for the connections to the beam in the last part - can they rotate and put the supports in a bind if the goop lets go? or do they have more than 1 rivet apiece?
Your Hail Mary - compressing the tube - Did you have any way to secure it in the clamp? I'd be concerned about it popping out (or snapping) given too much pressure.
Did you consider welding (or having someone else weld) the extra stock to the curved support instead of using the spreaders?

Thanks!