the paint on the car is about 20 years old, so is a good base. its hard and wont move at all. So your theory there is sound. I wouldnt bother priming it if youve got no filler work done, scotch the whole car and your good to go.
id prob give the bumpers 2 coats, then put them back on the car and spray it all in one go. as its a pearl, like my one it all need to be together to get the finish, or the pearl will sit differently if the panels are sprayed separate from the the car.
6 liters should be enough, with some spare.
The BMW wasnt thinned when we got it, and was mixed 1:1 but the paint just wasnt covering very well at all. We got it from our normal supplier and have never had an issue with quality before. but we had to get more and that was the same.