(on ur image, top down)
a span is the distance between 2 CV's

yes theyre the two opposing curves, they shud contain the same number of CVs in roughly the same place

both profiles shud have the same number of CVs as well

the direction, you can reverse the direction of the curve in one of the nurbs menus, cant remember which, sorry

opposing curves are just like ur two rails, they shud have the same number of CVs, and ur profile curves, i wud just define them as the same type of curve