Can anyone illustrate how, geometrically, the right elbow can serve as the swing center in a right arm swinger?
No, because it doesn't.
The Right Elbow serves as the Center of the Stroke only in the Bat Minor Basic Stroke, a 'Hitting' Stroke in which the Club is accelerated radially (from behind the Shaft).
With the Right Arm Swing, the Club is accelerated longitudinally -- lengthwise in the direction it is pointing -- with the Left Shoulder serving as the Center of the arc.