Jake1111, I've been following your recent posts, and definitely understand your frustration. I have a white coating that I can brush off most of my tongue, but would not come off of the middle back section no matter how much I resisted the gag-reflex to clean it. Recently, as in today, that spot has finally come off. I believe it has been helped by one part of my candida program. Actually it is the program I am using for SIBO. Now, I admit, I have not been to my doctor to test me for SIBO as I am an impatient person. However, I have suffered from very similar symptoms that have not altogether responded to my candida treatment.
There is however a simple test for it that a doctor, especially one as understanding as your doctor seems to be, can have done to deny or confirm the possibility. It is called a Lactulose Hydrogen Breath test. I posted about SIBO earlier, and I will attach the article about it for easy reference. But if indeed you believe that your issues may have another cause, this is one possibility. It can develop after Antibiotics , and it can affect your absorption of minerals, which can also affect your body's ability to produce HCl. Of course, low HCl could be the cause as well as the result.
Anyway, I figured that this is something that a simple test could likely determine, and that could get you closer to the answers.