Yes, C means Coulomb. It's a measurement of rate of charge.
Coulomb - Wikipedia, the free encyclopedia
I'm heading out, so I can't really reply to all of your questions... But, I believe that (if I remember correctly) MetroMPG already hit on the best way to assess the draw on your battery. Measure your battery's charge. Disconnect the battery. Drive for a given distance/time. Remeasure your battery's charge. From that, you should be able to derive how much electricity your car is drawing.