1st paragraph answered by your picture.
2nd paragraph, trying to condense 5 years (yikes!) of lurking in the Aerodynamics forum. Specific questions?
3rd paragraph — The part about
vena contracta is probably wrong. That describes what happens with a square edge, such as if the bellypan attaches to the edge of the airdam or bumper. If the bellypan attaches to the bumper and the airdam protrudes below that it's not
vena contracta just a straightforward obstruction as Panter140 points out.
It comes down to the relative height of the bumper/airdam and the engine and suspension. The bellypan should be flat, with a rolled up front edge if possible. That obtains the converging duct. The area in front of the tires is a different matter. You want spats or
sponsons there.