I will also mention that he would not stop for people who had tons of experience with horses either. It could take 5-10 minutes before, to stop him.
Tons of experience doesn't necessarily mean anything. I know people who have worked with horses for decades that I would not let ride mine. Its the really good horse people you want around, which by the sounds of it, these were not. I could get him stopped within 30 seconds, using one rein, not because I'm super talented or knowledgeable, but I have picked up some basic skills along the way that help me ride safely. A horse that wont stop is not safe. A horse that takes 10 minutes to stop is dangerous. On the trails I do, we would both be injured or dead.
Back to the subject at hand, its great that you are learning new skills, and applying them to your horse. You will have a better horse and a safer ride. Knowledge and keeping an open mind are key when it comes to learning how to communicate with your horse, and this post shows you are on the right track.
I highly recommend also looking at videos of a one rein stop, and teaching that to your horse as well. Its not something you use all the time, but its kind of like installing an e-brake in your car; you don't need it most of the time, but its sure nice to have when you do!