This isn't entirely true. When creating a "wide" version of a shoe, what is really happening is that they are increasing the girth/volume of the shoe. This can be achieved by heightening it or widening it. Very often, to go up one "width" a maker will simply make it higher, not wider. It is definitely worth getting the next width up and seeing how it fits.
Not in top quality shoes. Everything is graded like clothes. Good polos don't use the same size collar on the medium as they do on the large.