It's so hard to not go looking for the answer now that it's out there, but I've tried so hard to stay spoiler free, and it's just one more month. (Which means the following is based on the show, and issues 1-32.)
I'm operating under the idea that it will be someone from the inner circle, because Joss is Joss, and there must be trust for a betrayal. My guess is Giles. He's proven time and time again that he does what he deems necessary to keep the world at large safe. Murder isn't out of the question, and in times of war it's pretty much guaranteed. He's tipped towards the dark side, and all of this is reaffirmed in "No Future for You"
I haven't reread everything, but what I did see didn't go against my theory. It seemed that Twilight knew that Riley was a double agent so that would explain why he looked for the gang when he knew for a fact they were in Tibet. Buffy asks Giles if they are the good guys in Retreat Part 1 and he conveniently doesn't need to respond.
It could also make Ethan as Buffy's dream guide make more sense.