ok so Charon, Veronica, and Faire should be headed to a thieves guild contact to see if he or some one higher up could help them find Quirinus. i'm guessing though since this is the thieves guild they wont just hand the info over for free and want something in return. perhaps an item belonging to a official of some kind?
I'm not really sure. According to The Elder Scrolls Wiki, the Thieves Guild are found in the Ratways beneath Rifton. Where are we going to have the Thieves Guild in Windhelm?
And, perhaps more importantly, who are we going to contact? We don't really know any members of the Thieves Guild in game, so I've got no idea.
And I'd imagine the Thieves Guild would just ask for money. It would be a little odd for them to ask some random strangers who need information to do a job for them.
On the other hand, you mentioned that Charon has done a few thievery missions in his time. Perhaps Charon is known by the Thieves Guild and they want him to do a mission for them, since he's so well known for being one with the shadows.
Thoughts?