I'm sorry for not reading through all the posts, so here are my opinions.
~ If BCC means no one else could see the addy, there shouldn't have been any way Taylor COULD have known who the BCC was to. Could she have even known there was a BCC at all?
~ This e-mail is way off-character for Jonas. Has he responded about chatter about this e-mail?
~ I like the idea of Bree and Jonas playing against the others.

delightfully evil!
~ The response to the e-mail seems to try to put Sarah in a better light (e.g. making her seem smarter, correcting spelling, being the more mature one, etc) and SHE was the one who knew it was BCC'd to DB. Could she have something to benefit from this?
Edit: what if it's not so much Jonas and Bree not including the other three, but Sarah and Taylor working wiles on DB against the important Jonas and Bree duo? Perhaps some remnants of OpAPHID endure, such as the Lincoln quote of a house divided...