I told you I have a Sparco EVO2 with your name on it.

Seriously!
And just for you, since I bought a fancy new welder, I can make a bracket for you that will pretty much mount the seat on the floorboard.
To answer your questions:
1. You need the seat, side mount brakets, and floor mount brackets. I used a Sparco EVO, OMP aluminum side mount brackets, and fab'ed my own floor mount brackets.
2. You can have just one seat, but out of common courtesy, I'd have the same type of seat with the same type of harness for the passenger. I've heard the same rumors you have, and I have to say, if I were an instructor, I'd want to have the same safety measures as the driver!
3. Yes, a race seat can be higher or lower than stock. That's the beauty of side mounts.
4. Buddy Club? Sparco, Momo, OMP, Ultrashield, or Kirkey for me. Racetech is also good. Butler Built, if you have the cashola.