You can only see the comments between your friends. For example, if your friend replies to a comment from someone who is not your friend, you won't be able to see it. If your friends comment between each other, you'll see their comments.
Not if you use any of these methods! Other people can view comments, but these messages are private messages, like a personal text message between you and a friend. If...
It depends on what mobile device you have. Some devices show the comments section, others don't show them and there is no way to post a comment within that device.
