Old hat...

I have a backwards mounted 5" Mopar Bracket style scoop on the car with a hole in the hood even. The pressure well at the cowl puts air INTO the engine up to around 100mph.
Hmmmmmmm,
are you saying that the air under the hood is being forced down and out of the engine bay by this high pressure air?

No. A pressure well is a low(er) pressure, the reason cowl induction works. The low pressure in the cowl area develops due to the windshield.

At what speed does this phenomenon begin?

Phenomenon? I hear the scoff and see the sidewise glance from here...... ;)

I am saying that, if it's misting or raining, I can see drops from the top of the scoop opening get sucked into the engine compartment. Also, I have taped ribbon to the top of the scoop opening and have seen IT fluttering into the engine compartment.

Does it interfere with the air pressure differential across the rad which is the only way that the rad can get rid of the heat? Does it interfere with the carb calibration? or is the carb sealed to the pressure box? Have you actually measured the pressure in there?

I have no answers to the first two questions. What the scoop does to the pressure differential across the radiator, what it does or does not to the carb calibration I can't say. The folks that made the scoop (Harwood as I recall) said mounting it backwards was preferred, up to a bit over 100mph; backwards keeping detritus out of the engine compartment. It was their recommendation that convinced me to put it on backwards. They supposedly did the airflow testing to know what worked with their product. It is not a shaker situation...the carb is not sealed to the hood opening. I have not measured pressures.