I think we're going to have to agree to disagree. I've just reread your post (hey, I'm human and open to making and acknowledging mistakes) and I'll quote portions below again.
As you've phrased it there is 100% correct. What is incorrect is when you said:
I explained with examples how this simply isn't true. It's not like you're a plumber, a plumber, a plumber then after 30 years in the trade you become a plumber-nim. I would say "I'm Andy plumber, nice to meat you David plumber-nim".
If you mean Korean-Americans or Koreans living in America, they may have Americanised things because it's what their target market is used to, but outside of normal synonyms found in any language, the words consistently mean the same thing.
But again, from visiting quite a few dojang in Korea, that's not the case on these two common titles and the usage of -nim.
Again, I entirely disagree.
Can I ask, what is your reading/understanding level in Korean? Are you OK with Hangul, can you speak basic sentences, some level of fluency in being able to hold conversations in Korean? You say about going to Korean websites and see them used differently.
To tell you where I'm coming from, my Korean tutors have both considered me "lower advanced" in Korean, although I only feel comfortable saying I'm intermediate. I have a weekly lesson in Korean where we discuss current affairs more than actually "learning" and that hour is spent entirely in Korean - although I have to sometimes ask what a word means, but I do so in Korean and have the explanation given back to me in Korean. I've also been to Korea 8 times in the past ten years, have Korean friends there that I speak with in Korean while there and have hosted them here too and acted as translator on their last trip (previous times they've happened to have one of their team that is fluent in both, and I'm not fluent yet).
In case you don't speak Korean but want to learn by the way, I would recommend italki.com - it's where I've found my tutors and means I can have 1-1 private lessons by native speakers in my home over the internet.
Anyway, if I visit actual Korean websites (rather than American websites by Korean masters/grandmasters) again, the usage of -nim is consistent.
To revisit your message as you asked I reread it:
Kwanjang still doesn't mean anything about a leadership role (i.e. running a class on behalf of a master), it's about school ownership.
The only possible time I can imagine this happening in Korea is if a senior grandmaster (high rank and old) is addressing a young school owner. In general when a senior is addressing a junior, you don't technically need the "-nim", but that shows it's about respect between levels rather than "master tradesman or someone highly accomplished in their field". Two newly qualified 4th Dan certified masters would address each other as Sabeomnim because neither is junior and neither is really a master tradesman or highly accomplished at that time, they're both fresh newbies ready for their first teaching sessions.
Also, even though it's technically not incorrect, I've still never heard that. I've had VERY senior grandmasters in Korea (both that I know privately and Kukkiwon senior officials) call me Andy Kwanjangnim and Andy Sabeomnim, even though they technically have the right to drop the -nim. Not because of any accomplishments, but because it's rarely dropped and is in everyday language used when referring to someone else's title.
I assume you mean Bojo? ;-) (Bo-ris Jo-hnson)
Don't get me wrong, I'm happy to discuss this. It just feels that you're incorrect and I'm trying to help you understand what the correct situation is. There are a lot of topics where I'll shut up and let others talk and I'll learn. There are some like this where I try to provide correct information as best I can from the experience I've had.
"Everyone is entitled to his own opinion, but not his own facts" -- Daniel Patrick Moynihan
If
@Jaeimseu wants to jump in and correct me, he has even more experience than me in Korea and Korean culture around -nim and the title Kwanjang, having lived in Korea for a number of years and regularly trained in a Korean dojang. (sorry to drag you in to it and feel free to just ignore if you don't want to get involved)