1. Electric water heaters are almost "generic". They all use the same elements and thermostats, and they can ALL have tank failures within the warranty period.
2. Energy efficiency ONLY comes with more insulation around the tank so the wider the tank is compared to others the more efficient it will be.
3.See the answer th #1.
4. Connect the circulation line to the cold water supply INTO the heater. It will work exactly the same way as if it were connected to the drain outlet. And to answer your question, NO the do not.
5. Follow the manufacturer's directions.
6. With the new low flow faucets the self cleaning feature ONLY works when you run hot water in the bathtub, or use hot water in the washing machine, which few people do.
7. 50 gallon or 120 gallon, the electric demand is the same. They both use 4500 watt elements, although some are available with 5500 watt ones, but it still uses the same wiring.