Hmmmm... that is a conundrum.

Techland handled this problem in Dying Light by chaining NPCLifePlaces' and forcing Player1 to stay within a set distance of the NPC until they had run their set course. Maybe that's the extent of NPC coop play?

Is there a way to attach items to Player1? You could, in theory, attach an NPC defend object and force AI's to follow if a zone was attached? Let me mess with some stuff and see if anything comes up.

Try this:
Throw down an NPC and make sure it's class is set to "HumanAI". Skin it to prefs (m_AISkinName is what sets the appearence) and name it something you will remember (here we use CoopBOT1, 2 ,etc). Now toss down a LifePlace and set the character's stationary animations and link it in NPC's asset settings (m_InitLP <lifeplaceblahblah>).
Now to make the AI help you out...
Jump into quest-script and toss this in where applicable (probly at map spawn, but it's up to you):

",set-ai-conflict-side-multi _ pro_player
(tab in once....Steam loses indentation)
,ai HumanAI CoopBOT1
,ai HumanAI CoopBOT2
,ai HumanAI CoopBOT3"

Not sure how far they'll follow you around... but it should work. Test and let me know, dude!

Coop bots shouldn't be too bad, dude... let me look into this for a min. and I'll post-up if anything works for me.

Hey Homies! You can comment on phases by using "<<<text>>" (3x< in front 2x> in back) and it will compile without issues.
So here's it in use: <<<Pizle is tha rillest>>

Let me know if issues pop-up, but like I said previously, it compiles fine.