Well written. Good info.
One note: Most people are not “in the middle”. There is no “middle. There is extraversion and introversion. They use different brain pathways.
No one says “Some people are right handed, some are left-handed, most people are in the middle.” It’s the same sort of thing. Different brain organization. Different neurochemistry.
What you see that makes people _think_ there’s a spectrum (which could have a middle) is differences in outward behaviour. Less extreme or noticeable behaviour. Learned skills. Practice. Role-playing. Differences in time of day or interest (an introvert, passionate about a subject, will talk your ear off. A bred extravert will walk away.)