Just to be clear, the rules say a unit can respond to any number of overwatch triggers. So does this mean, for example, that if a friendly armour unit moves five hexes, and an enemy anti-tank unit has both range and line of sight on them for the entire distance, then that same AT unit can fire on the armour unit once for each hex the armour unit enters?
No. Once per impulse.
Are units that fire overwatch marked with a Fired/Firing marker?