Idle dropping when putting foot on break or turning on accessories.
 

This leads me to believe the BAC is bad?

Any ideas?

I am also wondering if the throttle plates are supposed to be completely shut when the car is first started at idle. To the point where light won't shine through the other side

Its normal for the idle to drop slightly when a load is placed on the engine. The BACV will only raise the idle a couple hundred rpm. There is a test procedure for the BACV in the FSM if you suspect it is faulty. .

Idle can also be affected by a leak in the brake booster. There can also be other items to control idle on the engine. On the S5 turbo, there are 3 different solenoids to control idle; there is the BAC, which you mentioned, as well as two that pull air from the intercooler when transient loads such as A/C, or power steering are encountered. Like he said, check the manual to find the test procedures for each device.