Owain,
I think your baulked flow beneath the wing picture, where the wing is within a fraction of a chord distance of the ground (Fig 4 of your diagram) is effectively describing a sort of `ram-air hovercraft', where there is an enhanced static pressure beneath the vehicle - plus a lot of extra drag, as you're slowing more mass down in the horizontal direction than you would a long way above the ground.
I would suggest that enhanced lift and reduced drag in what a glider would recognize as ground effect is a more subtle dynamic process where you slow air less horizontally, which allows you to move more air vertically, thus simultaneously increasing lift and reducing drag.
I agree completely that the surface pressure on the wing is related to the speed just outside the boundary layer, and any compression that's occurred. I'm just not sure the picture in the diagram is right to describe what I'm assuming about the flow around a wing in ground effect.