What advantage does using a move to block an approach give me versus just remaining in reserve?
The answer to that one could probably fill a book! One thing: maneuver attacks made from an approach against an enemy in reserve are unblockable. Maneuvers made from reserve are blockable.
Can any number of men block an approach? Can pieces blocking an approach and pieces in reserve conduct assaults?
Any number up to the capacity of a locale can move to an approach. However, only one piece may lead an assault from a single-width approach and two may lead from a double-width approach. Pieces in reserve may NEVER participate in an assault, their only option is maneuver attack.