Probably to late for your project, but you're on the right track. Remember that on an infinite wing the stall angle is a function of the airfoil shape only and is independent of aspect ratio. Of course, you don't really have an AR since the wing's infinite, but you get the idea. So it must be something to do with the interaction of the wing tips.

Think about this, too. Under some circumstances it's good to "trip" the boundary layer thus creating turbulent flow. The turbulent flow while creating more drag will "stay" with the wing longer. So you can think of the wing tip vortices as turbulent flow (because it is) and it will stick to the wing longer than a laminar flow before separating.