Firstly to answer Dennis - yes, the Lotus underbodies do work. The top aero guy there really does know what he's doing. Conversely, the Ferrari 360 & 550 have underbodies which are pretty but don't work. The 360's is too steep and the 550's is scuppered by poor management of the engine cooling airflow right at the front. It can be no coincidence that the Challenge Stradale 'race-spec' version of the 360 has an extra element inside the standard diffuser which makes it a whole lot shallower...
Back to browney - yes those pressure distributions are about right, though the pressure recovery from the entrance will be more rapid than that so there will be a peak there, similar to the one at the apex of the diffuser, before the gradual decay. Be aware though that once past that initial zone, the main reason the pressure increases is because of inflow from the side edges of the car. If you seal those edges (with sharper / deeper sills or, even better, full side skirts) then the pressure stays very low all the way to the back - hence massive downforce! The pressure rise is contrary to the boundary layer development on the car/floor which acts to constrict the flow further and so increase the speed and reduce the pressure, though in practice that's not really an issue.
The boundary layer thickness only becomes a problem at very, very low ride heights. You're probably only looking at maybe 20-30mm boundary layer thickness as a maximum, right at the back of the upper surfaces of the car but underneath the sizable acceleration of the flow thins them out much more, to maybe 5mm? A thinner one will form on the floor as well due to the mismatch between accelerated flow beneath the car and the car's travelling speed. Overall i'm confident that you won't get a viscous limiting of the flow under the body until you're down below maybe 10mm ride height (assuming you still design the inlet and diffuser sections appropriately).
If the underbody isn't smooth, then that distance increases, but then you're significantly reducing the chances that anything other than the shallowest of diffusers will work anyway.
To answer your last question, i'm an aerodynamics consultant by trade, though i tend to get dragged into various other areas of automotive development as well, if only as an extra pair of hands. I'm lucky in that i get to work on all different types of vehicle from trucks to F1 cars. The variety stops things getting boring and keeps you on your toes, as you are expected to know your way around each vehicle as if it was your whole life. Takes a lot of work but i wouldn't have it any other way.
