Getting a video would be the most helpful. That will allow you, as well as us, if you choose, to watch the trainer and the horse and their reactions to each other. It is very difficult to watch everything at once.
Interesting about the Mg and Selenium.......certainly something to try that won't hurt anything and may well help.
From the info from the place you got him, I would say that he definitely has LOTS of holes in his training, and personally, I would probably start at square one, based on that info, and NOT with a "rough, just ride 'em out" type of trainer, as it seems that was what he had the first time.
If you get a good trainer, and the one you have may not be it, you should be able to get through this.
Perhaps if you let us know what area you are in some of us may have suggestions. After returning from one of the Extreme Mustang Makeovers recently, I can tell you that amazing things can be done in 90 days. Yes, those were blank slates, so may be easier in some respects, but there are some really good trainers out there who may be up to the challenge. (and some may consider them the "cowboy" type.
) And they are all over the country.