Idiopathic Normal Pressure Hydrocephalus (iNPH): The patient's symptoms of gait deterioration, unsteady walk, progressive cognitive impairment, and the potential for urinary incontinence are classic for iNPH. The additional finding that would make iNPH the most probable diagnosis is urinary incontinence, as it is one of the key features of the Hakim-Adams triad (gait disturbance, dementia, and urinary incontinence).

Parkinson's Disease: Could present with gait disturbances and cognitive impairment, but the presence of cogwheel rigidity and essential tremor would be more indicative of this diagnosis.
Vascular Dementia: Could explain cognitive decline and gait issues if there were evidence of cerebrovascular disease, but the specific gait disturbance described is less typical.
Alzheimer's Disease: Primarily presents with cognitive decline, and while gait disturbances can occur, they are not as prominent early in the disease.

Subdural Hematoma: Can cause increased intracranial pressure, leading to cognitive decline and gait disturbances. It's a medical emergency that requires immediate attention.
Brain Tumor: Could cause increased intracranial pressure, focal neurological deficits, and cognitive decline. The presence of a tumor would depend on imaging findings.
Infectious or Inflammatory Causes: Such as meningitis or encephalitis, which could present with rapid cognitive decline and gait disturbances, along with other systemic symptoms.

Progressive Supranuclear Palsy (PSP): A rare neurodegenerative disorder that could explain the gait disturbance and cognitive decline, but it typically includes vertical gaze palsy.
Corticobasal Degeneration: Another rare neurodegenerative disease that could present with gait disturbances and cognitive decline, often with prominent apraxia and alien limb phenomenon.
Multi-System Atrophy: Could present with gait ataxia and cognitive decline, along with autonomic dysfunction and parkinsonian features.