Yeah, no addon can open the NPC window for you -- it has to be up for those options to work.
Instead of opening the NPC with the mouse via repeater, assign an Interact With Target key in WoW's Key Bindings menu (it's under Targeting). I set mine to "J". Then you want to walk up to the npc with everyone close enough, target it, make everyone assist you, and make everyone Interact With Target. Since they added IWT this is *the* method to use when multiboxing. It's also safer this way if you're on a PVP server
So for example, if you set it to J, you can make a key that when you hit Alt+J will both Assist and Interact with Target. It would go something like tihs:
New mapped key "Interact with Target" in "General" Key Map
- Hotkey: Alt+J
- No other changes needed on the Mapped Key's options page. Hold should be OFF (this is default on General key map). Use FTL Modifiers should be OFF (this is default on General key map).
- 1 Step. Actions in this 1 step, in this order:
-- Do Mapped Key Action: send "FTL Assist Me" in "General" Key Map, to Window:Current (this makes "Me" as in "FTL Assist Me" your current guy, so the other guys will assist you)
-- Keystroke Action: send "J" to Window:All w/ Current
Then you just run up, target the npc, and hit Alt+J. Win!