A lot of people don't like to be wrong. And the woman you mentioned from college, she could have had any number of things going on that were triggered when she was called out as wrong on something. To answer your question: yes, narcissistic personality disorder is on a spectrum, so it can be mild or severe. People can also have some traits or characteristics but not be diagnosed with it because they do not meet the criteria. Yes, NPD overlaps with other Axis II personality disorders, and predominantly with borderline personality disorder. And YES, there are a lot of times when the behavior that is outwardly showing, while it may look like a narcissistic action to you could be derived from something wholly different within the person. Some people just have anger issues, some people are just having a bad day and at their emotional capacity.

Thing is, no one likes being wrong. So normal people will generally get a little defensive when you point out they are wrong. People who are sensitive may get more defensive. People who have been made wrong a lot will be even more sensitive. Someone who is at their emotional threshold might have a meltdown. The person you mentioned with ADHD sounds like they were having a meltdown. In the case of your dad, it's a culture-centric behavior from his training. He probably CAN admit he's wrong if it were approached with respect. I have family in the military too and they don't like to admit being wrong but they will, eventually.

The key differences between someone having NPD and something else go back to the definition in the DSM-V. There are a whole slew of characteristics and traits that qualify someone with NPD versus just someone with an anger issue or someone who is too immature to handle being wrong. And I think that's another key thing: often the inability to accept responsibility or calmly handle being wrong comes from immaturity or lack of maturity. NPD is more than just immaturity. Someone who simply lacks the maturity to deal with their mistakes will often calm down and be able to be reasoned with and talk about it. A narcissist never will. Someone who lacks maturity can gain maturity and insight. A narcissist cannot.

I know you said you are trying to distinguish one from the other, but I think it's key to realize that either way, someone who doesn't accept responsibility for their actions is someone to maintain strong boundaries with.

Does that help at all?