1) Yes, any 9" ford diff can be narrowed. You might want to check with someone like John's Rear Ends for the price of a new one, built to your width.
2) 31 spline axles are significantly stronger.
3) Yes, you can change out the pumpkin, but all you need is a new ring & pinion, a crush sleeve and a set of shims.
4) I have no idea.