QCan I summon Kagemitsu, Lost Samurai from my EX area with Amelia, Sunny Paladin's activated ability?

AYes, you can. Note that Kagemitsu, Lost Samurai would retain its fighting spirit counters if summoned this way, and it would not evolve, as its automatic ability would not trigger.

QIf an Officer follower is put onto my field during my opponent's turn, does this follower's "whenever an Officer follower is put onto your field" ability still trigger?

AYes, it triggers regardless of whose turn it is.

QIf 2 Officer followers are put onto my field at the same time, does this follower's "whenever an Officer follower is put onto your field" ability trigger 2 times?

AYes, it does.