StarCreator

Well, this raises questions I didn't know I had. Does she gain the abilities of the person she's swapped with? Like, is she able to transform like Grace can? Would she gain Tedd's magic insight if she swapped with him?

This was my thought as well. This seems to me to be the first hint of Voltaire's motives out of all his appearances. I'm not sure we've been given enough information to figure out why eliminating Elliott would accomplish such a thing. But then again, we also don't know why Elliott is apparently also the key to Magus's leaving his current residence.

So I did a reread of the past few arcs and have started to wonder... we have no reason to doubt Disco Wizard believes he's the emissary of magic, but what if this belief is a deception? Perhaps on the part of another Immortal that we have known to be deceptive?

I doubt that is the sole reason for the marks; Rhoda and Luke are marked also, and neither have any significant relation to Tedd.

We've gotten more than enough information to know that it was no coincidence at all. Pandora engineered the entire situation. Pandora catches Magus after he falls into the main EGS world and convinces Magus that he has to follow her instructions in order to regain a physical body. Under her instructions, Magus influences Tedd into zapping Elliott with the TF gun, which causes them to visit the Dewitchery Diamond. The act creates Ellen and simultaneously awakens Abraham, who immediately sets out to find and kill Ellen. When Abraham eventually does find Ellen, she is defended by both Adrian and Nanase. Pandora intervenes to aid the latter, knowing Nanase will exhaust everything in the effort. When Mr. Verres arrives after the fact, he is furious at Abraham, but just barely not enough to kill him in cold blood. Pandora attempts to use Magus to change that - completing her quest for vengeance - but Magus refuses. Pandora even admits to Adrian, after the fact, that she "couldn't get anyone to kill the kidnapper", implying it was one of the primary objectives of the entire exercise.