Yep, that exhaust rubber does seem to be missing. I imagine that would cause stress on the exhaust so I would replace it before too long. I don't think you need to worry about getting a genuine Nissan part, a generic one will be fine as long at it's the right size.
I remember someone taking off the arch extensions too. They are the only person I know to have done it though. I seem to recall that it was just an unscrew job with good paint underneath - that was on a new or very young car though so you might need to give the paint a bit of attention to clean it up,