"Dorkus, you made 4.5 valid points, and are obviously knowledgable about the subject. But you called Paul McCartney a marketing gimmick, and I had to call you on that. "
Too bad you are wrong.
"Plus, for a way of playing to be “not correct”, in my book, requires some proof that playing the instrument that way somehow hinders performance. "
You can also play a flute with it extending to your left instead of the right. Or a recorder with your right hand on top. Have you seen Micheal Angelo? He plays the guitar left and right handed, overhanded, underhanded, all really fast. He is a total marketing gimick.
"As one of two guitarists for one of the two most popular guitar-orientated bands in music history, I offered Paul as an example that this might not be the case. "
You didn’t understand the point. You can play the instrument however you want but he isn’t playing it the proper way.
"“Boy, I wish I learned to play the guitar right-handed, because playing left-handed is really holding me back,” then we can start talking about ‘not correct’. "
Go get a flute and tell the teacher that you want to learn to play it with it extending out to the left. I assure you the flute teacher will laugh at you. While you are at it, try to find a left-handed flute. Then try doing the same with an oboe, a violin, an upright bass, a cello, a piano (LOL left handed piano… with all the high notes on the left), etc. You will get laughed at by any serious teacher.
"Playing left-handed does, possibly, hinder your ability to be taught ‘proper form’ and such from a right-handed teacher, which is another argument in favor of the kid in the OP trying to learn right-handed first. "
No, playing left-handed means that you spend needless money on a more expensive instrument that isn’t worth it. I can play the guitar left-handed and still play similarly to how I play right handed but it is just a gimmick that marketing analysts found they could make money off of especially when showcasing Paul McCartney and Jimmy Hendrix.
"And I did equate “not know what he’s doing” with “did not know how to play”. Sorry if that wasn’t how you intended it. "
Then you aren’t reading. Self taught people by definition start out not knowing what they are doing and eventually develop all the bad habits because they never have anyone critique their playing. If he knew how to play when he started he would have held the instrument properly. He obviously didn’t and then a movement of left-handed guitarists were born. When I was researching the history of the instrument I found that that they didn’t exist much before the 1960’s and definately not before the 20th Century.