A1: Basically, you do not. There are too many layout dependencies to accommodate form modification by users. You could probably do what you ask with a custom theme, but that would be major surgery and well beyond the scope of the forum.
A2: You don’t. Delivery addresses are picked up on delivery notes. If you want both addresses to show on a sales invoice, you will have to put them both into the billing address field (which you can do). But they won’t show beside one another, they will be vertically stacked.