In some ways that all feels like too much information. I applaud you for doing what you have done already with keeping the prediabetes under control and probably also the cholesterol etc.

I've been a bit confused about the whole Ig thing so just googled and came up with the following:
IgM - is the first antibody that appears in response to initial exposure to antigen
IgA - mucosal immunity
IgE - plays important role in allergy and especially implicated in type 1 hypersensitivity reactions
IgG - involved in secondary immune response (after IgM)

Not sure if those simplistic explanations help or not.

I think I'd be uncomfortable too about treating each symptom and not getting at the root cause of it, and probably want to hold off on supplements, unless they were ones already known to have fairly useful benefit.

I can't do fish oil either for the gut effects. Possibly have a tendency to react to it (though not a true allergy) - come from a family with frequent fish/shellfish allergies, so I'm pretty careful not to push the fish bit in my diet too much - just have the occasional plain white fish, and avoid shellfish.

I'd go with the "if in doubt, leave it out" theory.