I don't get it? You wrote a book on pool, yet you want to buy what is basically overpriced toilet paper, some cheap, low maintainance credentials? Since pool is so disorganized there is no point in having any credentials at all. By all means take an instructors course if you intend to learn from it, but if it is just to get you through the door, don't bother.
You basically want a meaningless diploma in order to teach, right? Why would you even want that? You go to a regular pool hall, I recon? Make friends with the house man or staff. Give them free lessons, show them something they don't know or can't do. Then have them refer people to you for lessons. Or at least have them confirm that you are an instructor when you advertise your services. Or do the same thing with some local players. If you want to build a reputation, be a bit selective with your customers. Teach the young ones with great potential for improvement, that way you can get partical credit for their success. You can always teach hopeless Harry who can't run 3 balls later. He's going not going anywhere, with your without your help. If you just want to teach for the enjoyment of it, then by all means teach Harry, but then I don't see why you would want the credentials in the first place.
If you can't show people something they can't do, or don't know, you have no business teaching anybody anything, diploma or no. I think you are quite a knowledgable fellow, so I can't see why you would want to be a part of basically a racket just to teach someone pool. Teach someone, then let the word of mouth do the rest or do some advertising at the pool hall to get you started.