1) Snipers have a course that they go through to get the sniper cards and paint. Rangers, and other SF units, go through the Obstacle course Wed afternoon.
2) Every unit does it differently but most have a basic rank structure. Promotions usually happen due to number of years played or performance on the field, or off the field.
3) Active military, and lots of former military, play. Just on my team alone we have 3 former military (Army, Navy, and Marine) and one current (myself, ARMY).
If you have more questions just ask or PM me.