Do they hold money on your card as a security deposit until you bring the car back? If so, how much?

Yes

I was only charged for the time of the rental. In my case only for three days. Plus a upgrade free of charge.

Yes, looks like the amount of full rental plus 20 per day.

Yes it was $200

Not sure.......I paid with a major credit card. It was charged when I picked up the car.