Ken,

Spanwise flow is simply not the cause of tip vortices. It is a largely incidental local flow phenomenon. If you don't like the earlier thought experiments, imagine a lifting, finite span wing with a huge tip plate. Huge enough to push spanwise flow as far from the surface of the wing as you want. Will there be a tip vortex? If there isn't, tell me how the air provides the reaction force to the wing.

banktoturn