You should be able to squat pretty low since on a steep slope you want to be able to get low when you need to, however there really isn't a standard answer since it just depends on the person, basically you don't want your binding placement to inhibit your ability to squat.. so, while at home, play around with it a bit and practice squatting from where you put the bindings, then you can get a feel for what will work best for you, play around with different positions until you get something that feels right (as far as the width)
It depends on how flexible you are. In a squat you need glute and hamstring flexibility. Your stance shouldn't be limiting how low you can squat unless you're in a forward stance, and even then it shouldn't be a significant difference.
Have you ever done proper back squats with perfect form (not that there is really any other way to do them hehe)? The same rules there apply to your snowboard.