I experienced just what you described about a year ago. I had the white coating on both sides of my checks and in the back of my throat. My gums had a fire red look to them that burned when I ate. They sent me to an oral surgeon who did a biopsy that was sent to the University of Iowa College of Dentistry. The biopsy came back as Lichen planus. I understand your frustration since I am a diabetic also. I hope this will give you another avenue to check into. Good Luck!