Chronic Heart Failure (CHF): Given the patient's history of hypertension, hyperlipidemia, aortic aneurysm, and ventriculomegaly, along with symptoms of lower back pain, unstable blood pressure, and intermittent swelling in both lower legs, CHF is a highly plausible diagnosis. The recent hemoglobin A1c of 6.5% also suggests poorly controlled diabetes, which is a risk factor for CHF.

Autonomic Dysfunction: The patient's unstable blood pressure and heart rate of 35 to 45 could be indicative of autonomic dysfunction, which can be associated with diabetes and other conditions.
Chronic Venous Insufficiency: Intermittent swelling in both lower legs could be due to chronic venous insufficiency, especially in the context of immobility (wheelbound status).
Degenerative Disc Disease or Spinal Stenosis: Lower back pain in an elderly patient could be due to degenerative disc disease or spinal stenosis, which are common conditions in this age group.

Aortic Dissection or Rupture: Given the patient's history of aortic aneurysm, any acute change in symptoms, especially severe back pain, should prompt an immediate investigation for aortic dissection or rupture, as these are life-threatening conditions.
Infection (e.g., Sepsis): Elderly patients can present with atypical symptoms, and infection should always be considered, especially if there's a sudden change in condition or if the patient shows signs of sepsis.
Pulmonary Embolism: Although less likely given the presentation, pulmonary embolism is a condition that could lead to sudden death if not recognized and treated promptly.

Amyloidosis: This condition can cause heart failure, autonomic dysfunction, and other systemic symptoms, but it is much less common and would typically require specific diagnostic testing for consideration.
Multiple System Atrophy (MSA): A rare neurodegenerative disorder that can cause autonomic dysfunction, parkinsonism, and cerebellar ataxia. It's a diagnosis of exclusion and would be considered if other more common causes are ruled out.