I think it's Kell mostly for meta reasons: these books are also intended for normal people, remember. People who haven't known for three books that Kell was a Cognitive Shadow and thus been half-expecting a return appearance at some point. People who this will be a huge twist to, instead of "oh *that's* what Kell's been doing". People who need all those clues to remember who that guy was again. It only seems so obvious it must be a red herring to us, because we're obsessive. (Same/similar deal for why I think Trell is Odium.)
Also, I've been advocating a kandra-possession Kell theory since around AoL, link's in my sig (though it doesn't account for the spike, I may need to rework it.)