Nice to see the edit come back !
I see you have removed Messages again, not enough used I guess...
Email contact form for private messages will be enough, I think.
Probably yes. Or some kind of "private mentions" maybe? You would start your post or write your mention with a special character, something like @@lucian, then the post and following replies would only be visible for the two users involved... That idea just passed over my head... :) Probably not the best to implement though.