6/12 is half the norm. I have 20/20 so I can see the smallest letter on the chart twice the distance from where you would see the smallest letter. 6/9 is pretty much using the same principle.
Not too sure of the rules but I think as long your vision can be corrected up to the required standard then you'll be fine.
That is correct.
Many of my friends wear glasses and they can see just fine. It's just a safety thing for having the requirement of 20/20.
Best of luck.