1. Whatever is safest and easiest to work around. You don't want shit hanging all over the place. I would think that putting the fan before the heat source would perhaps make the fan last longer, but if using a fan rated for the heat it doesn't matter. Why not one on each end, lol?

2. There are planty of things to worry about with growing cannabis...a few lost lumens from an HID throwing-out over 50,000 of 'em isn't tops among the important issues. After all...it is a Sun System to begin with. (cheap and effective, but not the industries' finest)

Use the safety glass, or wear UV sunglasses and long sleeved shirts. (limit exposure)

I used to use my 400w MH in my 4 x 2 x 7' tall closet with no issues, once you figure-out ventilation and light leaks.