You are correct in your assumption that your body is completing the circuit, however 12 volts is very low. It is a combination of high voltage and high amperage that gives you a shock. Think of it like water pressure. A squirt gun will just make you wet while a fire hose will knock you off your feet, and a really high pressure line, like a broken pipe in a submarine or an industrial pressure cleaner, can tear the flesh off your body. Also, while your body is a conductor, it isn't that great of a conductor. Metal is far better, as you see if you short out the terminals of the battery.
Answering the second question, the car battery is ground through the car chassis, and the negative terminal connects straight to the chassis also completing the circuit, this is why people always say, check your terminals and connections if having problems with car batteries.
Answering the second question, the car battery is ground through the car chassis, and the negative terminal connects straight to the chassis also completing the circuit, this is why people always say, check your terminals and connections if having problems with car batteries.