Nutritional Deficiency (Vitamin B12 Deficiency): Given the subacute onset of spastic weakness and numbness, particularly with exacerbation by alcohol, a nutritional deficiency such as vitamin B12 deficiency is plausible. Alcohol abuse is a known risk factor for vitamin B12 deficiency, which can cause subacute combined degeneration of the spinal cord, leading to the described symptoms.

Multiple Sclerosis (MS): Although the negative imaging and serologic work-up make this less likely, MS can present with a wide range of neurological symptoms, including spastic weakness and numbness. The subacute onset and the fact that symptoms are exacerbated by alcohol (which can worsen MS symptoms) keep MS in consideration.
Spinal Cord Infarction or Ischemia: This could present with acute or subacute onset of symptoms, depending on the extent and location of the ischemia. However, the lack of clear imaging evidence makes this less likely, though not impossible, especially if the infarct is small or in a location difficult to visualize.
Infectious or Post-Infectious Myelitis: Despite the negative CSF and serologic work-up, some cases of myelitis might not have clear diagnostic markers at the time of presentation. The subacute progression and the specific pattern of weakness and numbness could still suggest a myelitic process.

Spinal Dural Arteriovenous Fistula (SDAVF): This condition can cause progressive myelopathy with symptoms similar to those described. It is crucial to consider SDAVF because it is treatable with intervention and can lead to significant disability if missed.
Cervical Spondylotic Myelopathy: Although more common in older populations, it's essential not to miss this diagnosis due to its potential for significant morbidity and the availability of surgical intervention.
Intramedullary Spinal Cord Tumor: Tumors within the spinal cord can cause a variety of neurological symptoms, including spastic weakness and numbness. The absence of imaging evidence does not entirely rule out a small or slowly growing tumor.

Subacute Combined Degeneration due to other causes (e.g., Copper Deficiency): While less common than vitamin B12 deficiency, other nutritional deficiencies like copper deficiency can cause similar neurological symptoms.
Hereditary Spastic Paraplegia: A group of genetic disorders characterized by progressive spasticity and weakness of the legs. The age of onset and family history would be critical in considering this diagnosis.
Adrenomyeloneuropathy (AMN): A form of X-linked adrenoleukodystrophy that primarily affects adult males, presenting with progressive spastic paraparesis and sensory disturbances.