I've frequently thought about it in terms of whether people consider the "average" stimuli to be potentially harmful or harmless - something that would take seed early in someone's childhood (with potential genetic factors as well). An "introvert" or "extrovert" would just be a heuristic to define someone's general state of mind - I know many introverts who have extroverted personalities in certain situations, and many extroverts who act like introverts in other situations.

Not exactly. It has more to do with how deeply you process the stimuli, at least in a lot of situations. For instance, I enjoy parties, but then I think off and on for a few days about what happened, like what I learned I could make use of, what I would like to say to each person if I met them again, and so on.

If I have too much novel social stimuli, then I get a backlog and don't want to deal with people until I have time to process it. Extroverts, on the other hand, seem to get all done right in the encounter and are eager to go right on to the next one.

I'm not entirely sure what you mean by this, but the name of the phenomenon is arbitrary. It doesn't really make sense to say that it should be called something else.