It would differ by promoting the whole blog instead of just a post from the blog. The channel should be as the post promotion channel is or people would find it by typing blog and for the 3rd question depends on the person and how attractive the message is.
I dont think it serves any purpose. There's no point in posting the whole blog and loading up the chat servers when the whole point of the platform is to keep the content on steemit and connect with users on the chat. :)