Ken,

An infinite span wing can indeed shed vortices. Just stall it and watch. What it cannot do is trail vortices, as the Helmholtz theorem requires a finite wing to do. Having said that, I read through the NASA page you linked to, and I see how the notion of balancing downwash is being used. This is a reasonable explanation, and I should have read the text before making my comments. Sorry about that.

Spanwise flow is simply not required for induced drag, although it is almost certain to occur for a lifting finite wing. To prove this to yourself, just ask whether a wing with no spanwise flow could generate lift, and consequently generate induced drag. The answer is yes. You may not be able to build such a wing, any more than you could build one with infinite span. As I suggested in a previous post, one could run a computer simulation of a lifting, finite span wing, enforcing the condition that flow on the surface of the wing be chordwise. This wing would generate trailing vortices, as any lifting, finite span wing must, and consequently would suffer induced drag. My original point on this topic was that it is invalid to think that the tip vortex can be weakened or eliminated by preventing spanwise flow, or by making it harder for air to 'spill' from the bottom to the top of the wing. The tip vortex is simply the trailing vortex, which has the same strength as the bound vortex. To weaken the tip vortex, you must reduce lift. Methods which successfully reduce induced drag do so by reducing the effect that the tip vortex has on the wing, generally by pushing it's center out.

banktoturn