At this point, neuropathy is not a concern as a cause of the weakness. Even in the unlikely event that the biopsy is positive, this would indicate a small fiber neuropathy, which would be a sensory neuropathy, not a motor neuropathy. So, it would not cause the weakness that you have experienced.
Certainly, a lumbar radiculopathy can cause leg weakness, although the diagnosis would also ba based on weakness in the distribution of the affected nerve and changes on EMG within that distribution. This is why the reconciliation of the EMGs is so important.
A myelopathy also can cause leg weakness, although the ventral pressure on the spinal cord is not the usual location to explain unilateral leg weakness. Again, though, it is the findings on neurologic exam and neurologic studies that would indicate whether the weakness is from the myelopathy.
Of course, it is also possible that any other condition that places pressure on the nerve anywhere along the course of the nerve can cause weakness, but there has not been any evidence in any of the clinical information that you have provided that would indicate any of these other conditions.
Heartburn is typically a sign of acid reflux, also called gastroesophageal reflux disease (GERD). This is not a typical side effect to lithium or alpha lipoic acid. It is a common side effect to many different pain medicines, particularly anti-inflammatory medicines. It is also commonly exacerbated by stress or other significant illness as a non-specific response to that illness.
It is common for GERD to be worse at night, as gravity is no longer helping keep the contents in the stomach where they belong. The usual first-line management of heartburn would be the use of a medicine to suppress stomach acid, such as ranitidine or omeprazole.