You have had great results from your two surgeries for nerve and cord compression. I am surprised that you were able to go back to full activity after six weeks from a posterior neck fusion. Time to fusion for a posterior neck takes at least three months. Well, obviously you did well so no need to focus on that surgery.
A pars fracture in the lumbar spine after a decompression surgery is not highly unusual. The initial surgical decompression thins out the pars and if too thin, a fracture occurs. This fracture can cause all the symptoms you describe. Do you have most symptoms with standing and walking or with lying down? I assume your pain is mainly left leg oriented.
The pars connects to the inferior facet at the level of the fracture. Without a pars to support the weight of the vertebra, the foramen collapses and the nerve root is compressed. Also, the fracture causes inflammation and the nerve is right in the path of the inflammatory process.