Yeah I would allow trained Religion before I would allow trained Arcana, thinking about it now. Funny, there's another thread going on about underused skills, and I think you're on to something with Arcana bullying Religion and maybe even History.
That aside, I really like the cacodemon. The excerpt also mentions Kostistichie is in the book (and hopefully even more demon lords) and refers to exorcisms and alternate means of handling possession (the idea, as Scribble said, of demon-possessed items or tossing one into an inanimate object or something is raaaaad).
Kind of curious, though, why in the list of previews they opted to keep Incubi demons after having switched Succubi to devils. I bet it has something to do with Graz'zt. In fact, I wouldn't be surprised if in this edition Graz'zt is really half-demon half-devil or something. That's how I'm using him in my setting, anyway. He just doesn't quite seem demon lord enough for me, not in a setting where we have suave, handsome and charismatic archdevils. Unfortunately, it suddenly occurs to me that Malcanthet and Shami-Amourae won't be in the book, either.