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.
Dr. Corenman