The answer to those three questions combined would be in the form of a book.
You won't need the entire underdash harness from the 94, but, what you will be needing is the aforementioned book, which, fortunately, Ford has already written for you. You need the factory shop manual for the electrical/vacuum systems in a 94 Mustang, and, with that, you can go through, one system at a time, and see what is needed, what is not, which wires go where, all of that stuff. That's why I have all of the factory manuals for the vehicles that I drive. I don't actually need them for everyday car, a 64 1/2 Mustang, but, I also have horrible weather/ heavy load vehicle which is an 87 Bronco II. For that one, I have to refer to the factory manuals occasionally. I had no idea that you needed a special tool to replace the ignition module, for example. It's held on with two screws, but you can't see the screw heads, so there's really no way to figure out that these screws have these goofy looking heads that no screwdriver will fit.
I don't know how comfortable you are with working on automotive electrical systems, but, I do know that you are about to get a whole lot better at it.