Whizbang Dustyboots
Gnometown Hero
I don't think of demons typically being subtle enough* to work towards breaking up a would-be utopia through political in-fighting, which is what pitting two of the Concord Worlds against one another could potentially do, but you're right that there's already a demon involved in San Citlan. Good thought.Does it need to be a Devil or could a Demon Lord work? Pazuzu is an obvious choice since he already ties into the San Citlan adventure
(* Graz'zt is also pretty subtle, when he's got his pants on and is thinking clearly, so he's another good possibility.)