"There was dancing now on the canvas in the garden; old men pushing young girls backward in eternal graceless circles..."

"... superior couples holding each other tortuously, fashionably, and keeping in the corners — and a great number of single girls dancing individualistically or relieving the orchestra for a moment of the burden of the banjo or the traps."

Tortuously, fashionably... on the canvas in the garden, keeping in the corners (of the canvas)...

So there's a big square of canvas, maybe on top of the grass, that makes a dance floor, and the couples, with the women going backwards, don't overstep the boundary, whether they're young and pushed by old men or part of a couple of superior dancers. And then there are the females who dance alone. Individualistically or... what? They are bending over the knees of the drummers in the orchestra and allowing themselves to be strummed or beaten upon like a banjo or a drum. Or do you think they oust the band-member and do some drumming or strumming themselves? I'm thinking the latter. But the first image I got from today's "Great Gatsby" sentence was of single ladies becoming the instruments, offering up their voluptuous buttocks as a substitute for conventional instruments. On reflection, I merely see them grabbing an instrument and pretending to be a member of the band.