Would spinal fusion for spondilolisthesis help numbness and pain in legs and back and herinated discs?

Most likely not: Fusion help to stabilize the bone which keep sliding forward, numbness happen sequence to pressure on the nerves from the disc.

90% success rate in : A non smoker in terms of symptom relief. This is assuming you have failed a rigorous course of non op care including: medication, physical therapy, possibly bracing and even pain management in terms of spinal injections.