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A wing converts drag to down-force, more angle = more of both. Too much angle: the wing stalls and stops helping (but steals huge power). Too little angle: may even create lift, because the angle is NOT to horizontal but to the natural path of air coming off the roof. This is generally slightly down, so even a wing horizontal to the ground gives some drag.
I would describe a "wing" on cars as a negative lift device. And the "spill"/"end" plates mentioned on a wing are more to prevent air from leaking underneath to the bottom low pressure side of the wing. A "spoiler" has "end" plates to keep the high pressure above the flat? surface upstream of the spoiler. A wing is designed for negative lift vs low drag up until as you mentioned stall occurs. A spoiler is all about drag increasing with downforce.
And since most of the devices discussed above are behind rear axle, any rear downforce causes geometricly similiar front end lift, with no other changes.
And regarding the Mercedes 55 flap, bet it sure destablized the car at speed suddenly say in the rain when the brakes were stabbed
Reality check, that half the population is smarter then 50% of the people and it's a constantly contested fact.
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