Let's deal with the last question first. It's easy enough to turn off comments in WP. So - as someone else was talking recently about doing, you can turn off commenting, cross post to the topic and then direct people to the forum topic for the comments/discussion.
As to cross posting every post and comment to each other I rather suspect this would have minority interest. Why duplicate evrything? But thats subjective. The forum posts are saved in the file sf-post.php. In there you will find the code that creates the blog post. It would not be that hard to put code in to create a comment should you wish. Tnere are also hooks in the commnenting system I believe that would alow you to do the opposite. Take a look at the WordPress Codex, find the Developers section and look up the API.
V3 of the forum also has a new hook that you can use on every save of a post which has the post content passed to it. A good place to send that content on should you wish to do so.