That's an intriguing idea. I don't even need to have Irontooth send a spy in to town. He can just liase with Ninaran. The only problem is that Douven won't have seen Irontooth's note from Kalarel prior to the PCs finding it on the goblin's dead body (assuming they manage to kill him). Rather than looking for the spy, perhaps he'll just start making discrete enquiries to find out if anyone knew who the gnome was, etc. This will tip off Ninaran, who will perhaps offer to meet Douven somewhere secret and she will then kidnap him (again) and send him off to the keep, then try to set up a trap with Irontooth ...
For this to work, though, I think I'll need to let the PCs go back to Winterhaven, then go out to the kobold lair again and have Irontooth evade them so that he can then try to trap them by offering to ransom Douven back to them in exchange for the mirror.