Downwash/Flirting
Owain,
To the first quote: I'd reply that Ms 39's explanation that there's `more upwash' isn't right, as there is in fact more downwash, or else there would not be more lift.
To the second quote: I'd reply that when you flirted with the idea that an increased downwards mass flux, and a greater flow speed over the wing, I think that's absolutely fine: the pressure on the wing is not just static. As ground effect works for very slow gliders, any worries about thermal equilibrium can't be essential either. The boundary layer details must change, since Sir Isaac demands his pound of flesh for the increased rate of change of momentum, and the only way it can be delivered is via changing the integrated pressure normal to the surface over the wing section: the flow changes, the pressure changes. This can only happen if the flow above the wing speeds up by more than the flow below when the underground image wing comes close enough to modify the flow field.