Can a food intolerance cause itching? and related, can a food allergy not respond to small amounts of the allergen (fine at <5 g, sick at >30)?

Yes and yes: Food allergy is not a trivial matter. Food allergy can change from itching one time to fatal anaphylaxis the next. It is important to have allergy testing, have a consultation with an allergist, avoid all foods you are allergic to, have an epipen (epinephrine) and an emergency action plan handy, and carefully examine the contents of everything you eat for contamination with the allergy triggers you have.