1) No, you wouldn't have to manually add it to each article. Ideally, we'd use the Joomla plugin 'content' plugin system to do this. The content plugins are fired whenever an article is displayed and can do 'something'. In our case, it would display the comment box below the article itself. Looking over K2, we may have to create a K2 specific plugin, but either way, we will. It's popular enough (and again we use it) to not create something specific for it.
Again, no timeline. But 3.0 stable is being released today which lets us focus on some of the more non-core functionality such as this.. and we can't wait!
2) We probably will only make K2 or content specific plugins where it makes sense. A fanbox isn't really article-specific. A 'like' button is though, so I can see creating a similar plugin for that.
Hope this helps. Again, if you have questions, you know where to find us