Vorticey, I didn't make up the circulation theory of lift, someone light years cleverer than me did that. It is one of 3 well accepted theories to explain the production of lift - the other 2 are: momentum theory and dimensional analysis.
It is a bit like the physics professor explaining the behaviour of light - sometimes it is easier to believe it is a wave and at othertimes a stream of discrete particles - which one is correct I couldn't say.
The ground effect phenomenon is easily demonstrated but, as we have seen on this thread, difficult to explain clearly and concisely.
The vortex theory is extended by the realisation that you cannot have a vortex with open ends and the resulting Horseshoe Vortex is the end game of the circulation theory. It can be demonstrated by placing a flat piece of wood in a bath and tilting it to produce an AoA. Move the wood forwards and the vortex is formed at the trailing edge first indicating a circulation of water around the chord of the wood.
The effect of the building you are talking about is recirculation which is nothing to do with ground effect.
As to your ASI, the pitot pressure (the air rushing down the pitot tube) is made up of static pressure ie the measurable pressure of the air free from the influcence of the pitot; and dynamic pressure - caused by ramming the air down the tube. Inside the ASI you compare pure static pressure with dynamic pressure, across a diaphragm in your case, and since the static elements cancel each other you are left with a way to measure dynamic pressure by the deflection of the diaphragm.
I don't think anyone doubts that the pressure under the disc is greater than above it - the helo wouldn't fly otherwise, it is a question of how the ground influences it that is the nub here.
In a free air hover the air is being pushed down against air which has not been through the disc and will offer resistance to the moving air (equal and opposite reaction) - in a low hover the air pushes against the ground which also resists it (clearly more effectively) so the rotor has to do less work to achieve the same end. Is it a function of increased pressure though? Discuss....
Which is how this thread has become so long