Hi Whune:

1. good question! I never thought of it before. I think that technically they are the same thing, but that double-stops are usually referred to in a lead context.

2. to me the difference between an A and an A7 is greater than the difference between the two power chords you mentioned, but that's just me. There definitely is a noticeable difference in sound with those power chords.

your formulas look correct.