So let's say that the Elite Imperial Officer uses executive order to command BT-1 to attack. Can BT-1 attack via missile salvo, since that action is considered an attack (the text attack appears in the description) or is BT-1 limited to his usual attack? Also, regardless, he does get to add a green die before attacking, right? Since he is still declaring his attack that he was ordered to perform, right?

Missile salvo is a special action, and cannot be commanded to use it. It is still an attack that he is using so he still becomes focused before the attack.

Missile Salvo is a special action. It allows to perform multiple attacks. Missile Salvo is not an Attack action. Attack action allows to perform an attack. Executive Order does not allow to perform an Attack action, it allows to perform an attack.

When you perform an attack, you perform it with the attack type and attack pool specified in the deployment card (or if a hero in the campaign, with a weapon of your choice). Yes, Assassin gives focused before he declares the target of any attack.

(Missile Salvo contains performing an attack, so it counts towards the one-action-containing-attacks limitation of non-heroes.)