Last Saturday I paused halfway down a mogully section and someone on the chair asked how long my board was. I was on a 183 at the time... it's not the norm, but so be it.
It's been a long time since I rode anything shorter than 170, but I don't feel much difference switching between my 172 and 183 in bumps. I do prefer the 172, but I think that's got more to do with it being stiffer and narrower than it being shorter. I like the stability of a longer board in general though ("longer" meaning "over 160").
Narrower means better leverage on the edge, as someone mentioned already, and stiffer means that if my CG gets too far forward or backward I can muscle myself back to center and the board will support me rather than curl up under me and just let me fall.
For the "zipper line," I try to think of my front foot as going straight down the hill, and my back foot swinging around behind me, unweighted in the turns. The "straight down the hill" part doesn't happen nearly that precisely of course, but it's a mental model that works for me.
If you want to run a duck stance, my guess is that it's not going to be all that different except that your upper body will be sideways so that you can still get lots of rotation / counter-rotation. It's harder to get high edge angles on your heels, without bending a lot at the waist, when you've got your knees oriented that way. But if you want to handicap yourself like that, go right ahead. ;)