Nick, I am nearly with you in that I agree that the pressure IGE below the disk is the same as OGE and that there is not a 'cushion (cussion for Vorticey)'
of high pressure air.
I still maintain that an increase in pressure caused by the vortices being modified by the presence of the ground is the mechanism by which the Ground Effect phenomenon is created. Without an initial increase in pressure there would be no way for the lift to increase and no reason to lower the lever to prevent a climb.
BTW just to muddy the waters again, I went on a training sortie last night, set the QFE just before take off which gave me +10' on the Bar Alt. As I lifted to the hover the RCDI showed a RoD and the Bar Alt reduced to - 30' AND stayed there once established in the 10' hover. This is with a reasonable altimeter with a vibrator (although why it needs one when it's mounted in a Sea King I don't know).
DJ don't forget temperature in your statement about prssure and density eg if temperature stays constant then rising pressure will increase the density.