Some musings, not all things I might believe, but I like to let my mind wander:
Pot dealing: I've known more than a few dealers back in the day (small-time except for one who was A Big Deal kinda guy). Sure, JC kinda fits the profile of a small-time dealer. Delivery is a great front for that. Under the law, sure, a small time dealer is a criminal. But like all other things, if one person breaks one law, that doesn't mean they'll do more serious things (steal a car, murder someone). None of the small-timers I knew would have so much as shoplifted a pack of gum. The big time guy would have easily been able to murder someone over business. I highly doubt at JC's age that he'd have made it that big yet. Not that it's that way across the board, but I can't really see a "he sold some pot ergo he can kill" link as anything very binding.
The Other Guy Postulation: Okay, let's say there's another dude in her life. Let's say she did agree to run off with him. I find it highly unlikely that she'd leave EVERYTHING. Especially as a pet lover, her dog. She would probably have taken a few small sentimental objects and most definitely the dog. I doubt that was the night where she'd have been bolting. Maybe she met up with Other Dude and THAT is the relationship she broke off - and that's where things went badly for her. I don't necessarily believe this is true, but if it were, I don't think disappearing like that is logical or realistic.
One question of my own here - do we know anything about the boss/head-person-whatever at her internship who took the photos of her? Could be totally innocent, of course, but I'm just curious what the deal is with him/her. Something about that just kind of set me on edge.
Oh, and one other thing - I went to college in Oxford, about 30-40 minutes away . . . Have any search parties gone out to Hueston Woods? First place that came to mind. Big lake, lots of woods . . . you never know.