" which further suggests that different life pathways and environmental differences can change how one’s personality develops between the two opposite ends."

How does this work. I can see what it means that traits develop over time, and that a chosen path in life contributes to that. I do not see, how a occasional different social setting has an impact on the personality. It surely has an impact on the behavior, but the person is just not there long enough for it to have an impact on the personality. It would take years and years to develop the patters and anchor them so rigidly into the person to make them part of the personality, that i doubt anyone's work environment (as in the given example) will be intensive enough to contribute to it.
It would also mean that someone being diagnosed as a narcissist would fluctuate in narcissism on the scale it self from day to day, even hour to hour, on the spectrum itself. I think that would be unlikely ?