What is the death rate of addison's disease?

100% untreated. If treated adequately the response is miraculous. They require steroids. Diagnosis is not easy because the levels of cortisol change during the time of day, the stimulation test with syntetic acth is not widely availa. Present.May be acute or insidious, with substantial fatigue and weakness associated with mucocutaneous hyperpigmentation, hypotension and/or postural hypotension, and salt craving.
Hard to say. Deaths directly due to adrenal insufficiency (ai) are extremely rare, but having ai complicates other conditions and can increase their death rates. Example: if two people sustain identical injuries in an auto crash, and one has ai, they might be more more likely to die from those injuries than the person without ai. Same thing holds for people with serious infections or being treated for cancer.
100% Thankfully, addison's disease is rare in humans, though very common in dogs. However, untreated, it will kill the victim eventually -- and after years of being miserable, being considered emotionally sick, getting a "chronic fatigue" workup, and so forth.