Pars repairs only work well if the disc at the fracture level is intact. According to your MRI report, your L4-5 disc is degenerative (“here is desiccation of the disc space with mild diffuse disc space narrowing. Focal linear increased T2 signal in the posterior annulus at the 6 o clock position measure 8 mm transverse. Minimal broad-based disc bulge”).
You cannot put an artificial disc in a level with a pars fracture so that surgical plan should be discarded.
The plan should be at least fusion of the L4-5 level where you have the isthmic spondylolisthesis. The question should be asked if the L5-S1 level needs to be included in that fusion. It seems that all surgeons you consulted want to fuse L5-S1. I assume that this level (L5-S1) looks bad enough that no one wants to leave it alone.
You could undergo a discogram (see website) to understand if L5-S1 is a pain generator but if this level looks very degenerative, it may not be wise to leave it alone.
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.