Not very experienced in this area, but:
1) In vanilla, three way conversations are often scripted with timers controlling when an actor is to say a line. If you wanted to have NPC's butt in during player dialogue, that would be complicated. You could maybe add a result script to a dialogue option which closes all menus and returns you to gamemode, then activates your NPC to say something. Not sure if its really feasible though.
2) Again not sure on this, but I think you can create dummy NPC's to act as your talkable item, and simply have your activator activate the NPCref when the player er, activates it.
http://geck.gamesas.com/index.php/How_to_script_conversation_between_two_or_more_NPCsis the geck page on conversations, its very good.