R^2 is on the bottom. We don’t ignore the mass of one object because it’s insignificant, that would make the top of that equation 0 and the object wouldn’t fall at all.

That nifty gravitational law gives you the force of gravity on an object, not the acceleration. Force also equals mass times the resultant acceleration, right? So Fg1 = m1*A1 = G*M*m1/r^2 and Fg2 = m2*A2 = G*M*m2/r^2. m1 and m2 are present on both sides of those equations, respectively, so they cancel, and you get A1 = G*M/r^2 and A2 = G*M/r^2, which are identical. The mass of an object affects the force of gravity, but when you look at acceleration the mass terms cancel out.

They would have the same coefficient of drag, correct, but the air resistance would end up having more effect on the lighter mass of the hollow sphere, so it would be slightly slower to fall.

Archimedes principle here is accounted for in the different weights. Everything that you can put on a scale is already being acted on by Archimedes principle in air.