You can click "reply" to any post to make a normal post. It doesn't actually link / track back to the post you replied to.Generally yes... but in the event that the OP changes the name of the thread, it will display what the name of the thread was at the time you replied. Not that you really need to know that, but... you know.
To not have a subject, simply do not do anything to the automatically filled in subject. Leave it alone. It will not show up.