Welcome back to the forum, but sorry to see you found it necessary.
You have nothing wrong -- no NGU, no abnormality at all that I can see. And even if you had some version of NGU, it would not matter. NGU with no known cause (chlamydia and all the others you were tested for) is believed to be harmless. There are no known complications of any kind, either for affected men or their partners. But that's really beside the point, since you almost certainly don't have it. A bit of clear fluid in the urethra from time to time is entirely normal -- more common in some men than others, but not in itself a sign of disease or infection of any kind. I recommend strongly that you not see further exam or testing, and for sure that you not have an exam for urethral WBC.
To your specific questions:
1) Urethral WBC has never been a subject of careful research study. We know that presence of WBC supports a diagnosis of urethritis, but in itself is not abnormal and doesn't make the diagnosis. WBC without symptoms and with negative tests for no know pathogens is probably entirely normal in some men -- perhaps more frequent in uncircumcised men, but probably meaningless in anyone. If you were to have a Gram stain that shows WBC at this time, it would not indicate anything important and would not be a cause for further diagnosis or treatment. So don't do it!
2) The next step, as implied above, is to do nothing.
3) Probably such clear fluid is urethral mucus, not from the seminal vesicles. I am unaware of any evidence that it is more common in older vs younger men, and doubt it has anything to do with aging.
So from all I can tell, nothing is wrong and you have nothing to worry about. But if you remain concerned, continue to work with a physician knowledgeable about STDs and genital physiology, such as an STD specialist or urologist with particular interest in infectious diseases. But really, after all you have been through, the tests you have had, and the treatments you have received, I see no need even for that.
I hope this information is helpful. Let me know if anything isn't clear.