[Shinndou]

I think Horie Yui as Maria will be fine. She's well capable of doing creepy voices (latest example that comes to my mind: Kanako from Natsu no Arashi), and use a deep tone when necessary.

On the other hand, Kugimiya Rie as Shannon leaves me a bit puzzled. Sure she's a skilled VA, but I am not fully convinced mostly because one of plain simple reason: can she sound "adult"? My fear is that she could sound way younger than what Shannon looks. Fumiko Orikasa would have been more suited for this role, in my opinion.