"The starting cell is too dark". -> Not to sound like a [censored] or something, but I'm frankly getting tired of hearing about this one. You should take this up with the author of whatever mod is breaking the lighting system badly enough to interfere with a non-vanilla lighting source. I am not going to address this further, as all the measures I can take have been taken.
Do they have a certain day when they're at a certain location?? Because if so you could fast forward time and then move the player there??
They bounce around on a 24 hour interval. I've already tried to make a direct call into the script that controls where they're going but it has simply refused to force them to camp. The camp never shows up, and they're always walking from near Markarth on their way to Whiterun. I can't help but think this would have been trivially easy to force in Oblivion, but Papyrus is making things hell to figure out.
Anyone who might be able to help, have a look at the Caravans quest in the CK and its associated scripts. It's a much more complex system than you might think.