Two related questions:
to activate a system, is it always necessary to play a leader from your leader pool specifically to activate the system? Eg, if you play s leader in a system to complete a mission or to defend against an opponents attack, can that leader also activate the system?
Second, once you activate a system and move units it into and resolve a combat, is the system still activated (allowing you to now move additional units into it)? Or does one activation=one movement of troops? Say your imperial and you activate a system and move a transport and single storm trooper into a system. This reveals the rebel base. Can you then move more adjacent units in or would you have to wait until your next command turn to reactivity with another leader?