Well.. to answer most of the questions in the original post: It depends. There is not magic speed, RPM, etc that will determine all those answers. It's based a ton more on engine load. The ScanGauge can show you engine load, as well as tons of other useful monitoring values to help you get better overall mileage.
Heck, the main one is obviously instant mpg.
And as for your electrically related questions... no change in MPG
Oh... and it's as accurate as the PCM (cars computer) with delivered values, other then those calculated, which you can tweak at each fillup for more accurate MPG, etc.