I read your question carefully and I understand your concern. Unfortunately fusion while it may improve the symptoms on that level doesn t help with other levels, it can even accelerate their damage because the reduced mobility in L5-S1 after the fusion adds the strain on L4-L5. Hence it is not to be surprised to have protrusion on L4-L5 with displacement of L5 root. It does justify your symptoms, although to be sure about that an exam is needed (reflexes, muscle weakness, numbness location) to confirm symptoms correspond to that root and not to remains of old problem. As for remedy usually it is attempted with physical therapy and pain killers, local injections can temporarily help as well. If those are ineffective and symptoms persist surgery can be considered again. I hope things work out for the best.