Can addison, disease be misdiagnosed, along with the hyponatremia, as dementia?

Yes: Both hyper and hyponatremia can cause symptoms suggesting dementia.

Not usually: Addison's causes severe fatigue and weakness, loss of weight, increased pigmentation of the skin, faintness and low blood pressure, nausea, vomiting, salt cravings and painful muscles and joints. Dementia is associated with memory problems and loss of other areas of brain function. Certainly hyponatremia can cause confusion but the overall history of these 2 illnesses is quite different.

It could be: There could be a misdiagnosis if the physician did not take a full history, and did not check labs when doing the evaluation. The patient's thinking could be confused and cognition poor -- but hyponatremia can itself cause this. I can see how it could be overlooked in a very time-limited medical appointment, but most doctors would still want to find treatable causes of "dementia.".