The yogh was a real letter, but "insular G" is just a variant. Also the 7 isn't a letter at all as it stands for "and" (note I mentioned it in my earlier response), just like the shorthand "that" which the article mentions.
Notice how the 7, & and "long s", unlike the other examples, don't have majuscule forms? Yeah, I did too. It's because they're not letters!
Of course, the "ng" is just someone's attempt at a letter that failed, so I don't consider it a letter any more than if I created some fake letter and tried to get people to adopt it but they didn't.
If enough people use it then your letter will become accepted/real.
Feels you didn't get my pun, but whatever.