1. Hello, Let's say that: 1 npc is attacking me, 3 npc's are attacking my party members.

    Is there a way to get an array or sth of all those npc's, so I can get threat status of 'em?

    No, there's no general way to do it. You can get your target and focus, and your party member's targets and foci, but you can't get a lit of NPCs in combat. This is intentional on Blizzard's part.