Your information sources are mistaken. Pars fractures are almost non-existant with a microdiscectomy. In order to create a pars fracture, you would have to thin down the pars with a burr or rongier. The pars is generally not near the laminotomy (bone removed for canal access).
The need for fusion is normally about 1-10% if there has been a prior herniation and that need is normally many years after the herniation if at all. That means you have between a 90-99% chance of NOT needing a fusion.
Success rate for surgery for a standard disc hernation causing nerve root compression is 95%. The other 5% have chronic radiculopathy (see website for details).