I think the thing that everyone is missing is that you need a competent examiner.
Objective grading has lead to standardized testing in excess. It measures neither competency nor knowledge, just the ability to take the test.
Standardized testing is one of the main reasons AMerican education is not keeping up with the rest of the world.
Having said that...
The duty of the examiner is not to "weed people out" it is to make sure they can function at abasic level of a person with no experience.
If there is a reason to fail a student, it must be because of some obvious error. (Like believing you could palpate a diastolic BP) not because you think you wouldn't want this person in the field, to take care of your parents, etc.
It is the responsibility of the testing center to ensure their instructors are properly doing their job, with proper oversight, and a fair appeal process.
Rather than try to sue a hired proctor, you would probably have better luck demonstrating the school was negligent in their role and oversight which caused you actual damage.
Trying to sue a proctor over subjectivity is not only frivolous, I can't imagine any reasonable judge or jury seeing it as anything but sour grapes.
In this case, if I were proctor and you palpated a diastolic BP, you fail, period. Not because I said so, but because you have made an error which even the most inexperienced and basically educated people would not make.
As I said, the other stuff maybe just incidental things to improve on.
A proctor taking it upon themself to defend the one and true faith should be fired on the spot without pay and asked not to come back. There are no secrets on these tests. If somebody is failing students, word travels fast. If it is a national registry test, there should be a chance to retest the skill the same day.
I cannot imagine the NR rep which must be there would let that slide.
WHich brings us to the question, if the person failed the same station twice, with two different proctors as is the custom and asI understand rule, then only part of the story has been told.
In my experience it is also customary as a proctor to inform the instructor who should be there in person as a student advocate why the student failed and how to improve before the retry. That way it can be sorted out if it is anxiety or a deficency in knowledge training.
As an anecdote, I was once proctoring the intubation, combitube, peds intubation station of a NR paramedic practical. A student testing that day intubated and then told me he had never seen a combitube or other supraglotic airway before. He related his instructor said the technique was the same as the ET tube so there was no need to go over it.
The test was instantly halted, a meeting called with the NR rep, the instructor of the institution hosting the test (aka student advocate) and when other students confirmed this story, a training session was held to bring these people up to speed before testing.
They all managed to pass.