Can a Super Clamp fit on the bottom nub of the Avenger? I assume that's a rigid piece of steel? When the legs are positioned, does it spin or stay aligned with the C-Stand center column?
Yes it can:
However, the cap/nub on the bottom of the stand is a separate piece that can rotate:
It is quite strong and might be fabricated from the same 35mm steel tubing as the main column.