No, you cannot use an action card during a mission unless one of the leaders shown on the card is already in the system where the mission is occurring. The only exceptions are action cards that specifically move the leader to the system as part of their ability.

If both players want to use action cards at the same time, the current player resolves his card first.

Yes, you can activate a system that already contains one of your leaders. However, there is an important restriction: you cannot move your units **out of** a system that contains one of your faction's leaders. This means you can place an additional leader in the system and potentially move units **into** it from adjacent systems, but any units already in that system with your leader must stay there.

No, you cannot use a system's resource icons to build units if your opponent has a unit in that system. The rules explicitly state: "A player cannot use a system's resource icons to build units if his opponent has a unit in the system."

If a leader gains a second attachment ring, you remove the previous one. A leader can have only one ring at a time.

No, you do not have to reveal the mission card to your opponent when you search for it using an action card ability. The rules explicitly state: "If an action card ability lets a player search for a specific mission card, the player does not reveal the mission card to his opponent."

Yes, you can assign more leaders than required to a mission. According to the rules, "A player can assign up to two leaders to a mission, even if doing so is not one of the mission's requirements." The maximum is two leaders per mission regardless of what the mission actually requires.