Will have to have a look at that as answer to both of those questions is.... yes
http://www.mobilitywod.com/2012/02/daily-prog-shoulder-internal-rotation-13.html
It's not that it's a guarantee, but it increases the chances by quite a lot.
As fair as I'm aware, internally rotated shoulders/tight pecs are responsible for a huge number of shoulder problems.
If you felt more powerful on your other set, you probably got the form right. It needs to feel basically the same as your regular squat apart from you're deloading the weight onto the box.I'll have a play with things ice and post up a video on Friday.