I read the wiki article and few secondary sources. It’s something that anybody with pretty rudimentary music theory knowledge could figure out. It’s not that which is serving as the Nashville gatekeeper…
Probably not, but it’s likely that, if you show up to audition they will hand you one of these charts and say “play this”. If you can’t, you get booted.
Again, if your music theory is solid, and they say, play “5-4-1-4-5-1” or whatever, and they give you the key they want it in, I could probably learn to do that in an hour. Of course, I’m a music theory teacher, and the amount of music theory knowledge for a typical guitarist (not to be disparaging), can vary wildly. There are those that write and play their own fugues, and those that can’t read music at all at all.
The only really unfamiliar notation there is the following: The “-” after a number indicates minor (although this was a convention I was familiar with before knowing anything about the Nashville chord charts. My jazz teacher would notate Cm as C- for instance). The “<” symbol above a chord indicates a displacement of the beat an eighth note sooner. The diamond around a chord indicates holding a chord for a full measure. The underlines indicate groupings of measures (parenthesis are also used for this). The rest of it should be pretty self-explanatory notes indicating more specific rhythms in normal music notation.
I built the Wikipedia article on the Zeigfeld Follies composer Dave Stamper who never learned traditional notation. He came up with his own numerical notation using one through eight rather than C through B.
It almost got him killed when he traveled to England during WWI. Having sheets of papers covered with nothing but numbers was not a good idea, and he had to prove to a Judge that the numbers were music to avoid the same fate as a German spy on the boat.