I do not have the answer to your first question, but I personally would never use downshifting to slow the car down with a automatic transmission.
When you consistently downshift you are pretty much doubling the wear on the transmission clutch packs by using them twice as much. Brakes are much cheaper than the transmission. No matter how well-built a transmission is, its life will be shortened by consistently using downshifting to slow the car down. The only time I would ever downshift is when going down a long, steep grade, otherwise use the brakes.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record