An alternate hypothesis would be:
iron loading in the liver reduces HCl secretions in the stomach and eventually in the gut. Deficient HCl in the gut
causes incomplete digestion, huge proteins that resemble pathogens, massive over-growths of alien flora in the gut and absorption of huge proteins into the blood that are mis-identified as pathogens. The immune response would stay on high alert 24/7/365 while attacking the huge food proteins.
The iron in the liver reduces the levels of hydroxylated vitamin d which upregulates the autoimmune pathology.
I like yours better. Enough vitamin D seems to reduce a lot of autoimmune activity, and avoiding grains makes the gut flora more beneficial as well as keeping these large proteins from slipping into the bloodstream. I am going with your hypothesis.