Have you done any research on Jon Niednagel? He's best known for his work in sports, but he's also working behind the scenes to identify the 16 types through genetics. He believes we are born as one of the 16 types, and that it predicts not only much of our behavior and cognition, but even our motor skills (he usually types people based on the latter observations). He prefers the term "Brain Typing" to personality typing because people's personalities can change based on a number of things - upbringing, personal morality, brain health (neurotransmitters, etc.).
I agree that people can easily get caught up in the vagueness of function theory - how does one apply something that's pretty nebulous? I showed a lot of what may be termed "Ne" as a child, but some people here swear that my video appears Ni. I'm speaking abstractly as can be, so couldn't one come to the conclusion that I am, in essence, extroverting Ne and not introverting Ni?
Niednagel's Brain Typing has about the clearest methodology I have found so far, but it's still not completely provable. As a school teacher, I would say that the majority of my students are NATURALLY extroverted, spacy and daydreamy, not too worried about how they come across to others, and tend to be far more open-ended than organized, in keeping with Niednagel's theory that ENTP is way way WAY more common than MBTI fans think - and that their motor skills tend to be rather loose, etc. in keeping with his motor connections with type - but even so, it's not like we have a blood test right now that can confirm an inborn type. People can always counteract observations with ones that contradict them, after all - the brain is too complex to be strictly extroverted or introverted all the time, motor skills can be developed with practice, etc.
Whatever the truth may end up being about typology - we're just at the tip of the iceberg right now in neuroscience - I look forward to learning more about how the brain works. We have SO MUCH to learn.