Does the key to reversing allergies so as to cure even highly lethal sensitivities like severe allergic reaction to nuts, lie in requesting negative belief changes and soul attribute restoration of immune cells so they no longer view such foodstuffs as a foreign invader?

You are on the right track here in seeing, intuitively, this is a problem little different from what happens with many chronic illnesses, through alteration of cells and tissue activity from a corruption of makeup and wherewithal conferred by the initial configuration at the outset, when coming into life in a new incarnation. There might be built-in abnormalities conferred by the Law of Karma to replay an old drama and provide an opportunity for healing it more powerfully and effectively, and thus break a karmic repetition that is a challenge in life after life. Whatever the dynamics in play, what departs from divine alignment within the cells, tissues, and organs of the body represents a distortion comprised of negative beliefs within the cellular consciousness, and a compromise in the expression of soul attributes which together, define the wherewithal for normal growth and activity. When the soul attributes are suppressed or absent, the components of the body will suffer and this might be serious enough to cause death at an early age or at any time along the way, through bringing about a chronic, debilitating, illness or an acute syndrome that is lethal, like a heart attack or stroke.