Re: your second picture--
Pressure always acts normal to a surface. The arrows in the lower image are not correct.
A spoiler affects flow upstream because pressure is transmitted through air (fluid) at ~340 m/s. Fitting a spoiler may increase pressure as far upstream as the roof, as I found on my Prius with a small lip spoiler:
Gauge pressures with no lip spoiler
Gauge pressures with lip spoiler
Difference
Lip spoiler for reference
Les than $100 worth of equipment and a couple hours and you can measure yourself and find out what a spoiler does on your car.