I am not sure about the foot placement. I am of the opinion you should do what's comfortable. I think there is very little difference as far is how it targets the leg muscles. Just my take.
As far as back injury, I believe that occurs when the butt leaves the seat at the bottom of the press. Based on my observations this is a not uncommon gym goer mistake and a risky one.
"The fastest way to get strong muscles is also the fastest way to make them bigger. We know what works best to make them strong in the shortest time possible. If you want to get big, do the thing that makes them strong." - Marc Rippetoe
Zdravko Veselin Gaeta
I am not a powerlifter nor am I a bodybuilder. I just want to be big and strong.