1. Yes. Halfnut or Fullnut doesn't have any functional implication. You might stumble on to different revisions of the ROM code in a CX, but that isn't related to the nutty-factor of the calculator. (I have a fullnut CX with the latest rev. rom that i know of).
2. It might. Execution speed is pretty varying any way you put it. And people have speeded them up themself as well. I believe that you can speed the fullnut more, but that's a very vague memory.
3. Sortof. Nothing from a normal user point of view. But if you program machine code, the halfnut have some new stuff that isn't in the fullnut.
4. My fullnut is very happy with it self, and it's stuffed with every goodie you can think of.
Bascially, from a normal users point of view, the fullnut/halfnut comes down to a slightly different look of the segments in the display, and the rounded corners of the display. There is no other externally visible way of telling wether it's a halfnut or a fullnut. If you open it up, it's more obvious. And if you program in machine code, there are some new things to play with in the display, but that's it.