Inscribed above the Temple to Apollo at Delphi: "Know thyself" ...
A fascinating subject, and although it would appear to be well-nigh impossible, I believe that 50% of the answer lies in our genes, and the other 50% amounts to the degree time/circumstance have switched on (or off) those genes. Furthermore I would go out on a limb and suggest that our personality genes amount to no more than 12 in total, and are either dominant or recessive in each of us, eg extrovert/introvert is one gene, with which we're born either dominant or recessive. Leadership/follower would be another, curiosity/disinterest might be another ... I have about as much proof for any of this as Trump has for winning the election, and as Einstein warned, "Simplify, but don't over simplify" ...
As for Barrie, a fascinating question, but as hard to answer as it is to truly know oneself. Introvert? Well yes, on occasion, but according to Nico he could often set the table on a roar, depending on his mood. Although initially nervous (on delivering "Courage") he quickly evolved into a confident public speaker. Does that in some sense make him a closet extrovert? Are you an introvert? Am I? I've often labelled myself as an anti-social socialist = caring about people in the abstract but being bored silly by pleasant chit-chat ...
As you say, I think Barrie was more intuitive than a realist, though don't forget that the moral of "Dear Brutus" is that even if we were given a second chance, we'd all probably make the same mistakes again...
(btw, I did that test = apparently I'm INFP-A/INFP-T - sounds like a type of tropical disease