I understand how you feel. You brought up a lot of stuff in this post. I'll share my thoughts as best I can.
If you're asking is there grading discrimination, requiring us minorities to get a higher score than our white peers, no I don't believe that's a thing. I think the exams are scored equally for everyone. Now there is tons of documented implicit bias in all standardized testing. I'm not an expert in the subject, but it's been pretty thoroughly documented and researched, so I'm sure you can find the data if you're interested. So if you're asking if minorities are at a disadvantage. The answer is probably yes.
Regarding why passers don't get to see scores and transparency. The exam was also intended to be a pass/fail exam. If I recall correctly no one used to get any scores. Later NCEES started giving diagnostic reports to those who fail to aid them in preparing to retake the exam.
You asked about the pass rates for first time and repeat test takers. I'd love to know my score, but it is what it is I guess. I think there's a lot of reason for the pass rate, but I think much of it comes down to a combination of 2 things: "you either know it or you dont" and some people are better test takers than others. The data shows that most people either pass their first attempt or they have to take the test multiple times (i.e. more than 2). Most people don't pass their 2nd try. I don't say that to be discouraging; the statistics don't define you. If you didn't pass then you should attempt it again.
We actually recently had a pretty good conversation about diversity in engineering on this site, but it focused more on hiring and advancement than testing. Feel free to check it out.