may produce false-negative if user has weird privacy policy. Using signed request (tab applications) is only safe way to check if user is fan of a fanpage.
Pretty sure the second example is a violation of Facebook policy prohibiting pre-filled or app-generated content in the message parameter content. Yes, you can get the user to fill in the message content in a text box and use it, but I suspect using the same content to post many comments via an app will trip an alert on the banhammer.
Facebook's API is extensive. You can use FB as part of content management system—say, staff posts events, updates, photos onto Facebook page, and you periodically pull it all, cache, and display content (possibly with interactions) consistently with the rest of your website.
