bbodb, the reason we want to identify the setter is because she is the quarterback on the court. She is going to touch
every second ball for that team. We need to know if she is coming out of the back row if she decides to do a quick dump trying to catch the opponent off guard. If she executes a dump as a back-row player, while the ball is completely above the height of the net, she is guilty of an illegal back-row attack.
To get a perspective on 6-2, 5-1 and 4-2, study Section I - Offensive Alignments and Overlapping starting on page 81 and going through page 87 of the 2010-2011 NFHS Case Book and Manual. See if that doesn't help.
To answer your specific questions:
1 ) I track all six rotations on both sides of the net and I always try to know where the setter and the captain is on each team.
2) I will quickly ask my scorekeeper to "Give me the next three servers on the receiving team." That gives me the front row in order and, since I KNOW the setter's number, I now know whether she is front or back-row.
3) The NFHS Case Book and Manual as I suggested above.