Your symptoms sound nerve related but not all nerve related symptoms are from compression. There is a problem called peripheral neuropathy where the nerves become inflamed either from vascular, infectious or immune disorders. The symptoms typically increase in the evenings and the “burning sensation in the sole especially in the nights” is classic for this group of disorders.
The changes in your lower back MRI do not sound to be significant enough to cause the increased leg symptoms.
A fusion can increase the stress on the levels above and below but normally a fusion is designed to stop motion of a painful or unstable segment.
Donald Corenman, MD, DC is a highly-regarded spine surgeon, considered an expert in the area of neck and back pain. Trained as both a Medical Doctor and Doctor of Chiropractic, Dr. Corenman earned academic appointments as Clinical Assistant Professor and Assistant Professor of Orthopaedic Surgery at the University of Colorado Health Sciences Center, and his research on spine surgery and rehabilitation has resulted in the publication of multiple peer-reviewed articles and two books.