I bought my car a couple months ago, and did not know the prior service history either. I did the drain and fill my self to save money and time. but i plan on draining and filling every 30-45k. So the fluids in my tranny should stay pretty fresh.
Go for the flush for peace of mind. But imho, drain and fill should be adequate. The service manager at courtesy Nissan said it is not recommended to flush and fill. but i'm not sure if he was just trying to "sell me" on the drain and fill they offer.
The 12 qts is about right. From what my mechanic buddy told me, is that they flush, vacuum and fill all at the same time. They use a machine to extract(vacuum) the all the fluid from the torque convector while your car is running and they are also replacing the fluid at the same time. They use the extra fluid you bought to help flush the system.