@ChuckD, so many questions: MRI scan? EMG of lumbar spine and right lower extremity?
weakness of lower extremity, difficult to stand/walk on toes or heels? Atrophy of calf, glute or quads? Lumbar xrays; lateral neutral, lateral flexion, lateral extension done weight bearing?
Pars defect, bilateral or unilateral is pretty common. If the L5 vertebra doesn’t translate forward or back more than 4 mm during the lateral flexion/ extension of the lumbar spine, the segment is not considered unstable. What grade is the spondylolisthesis, probably GRD I. The nerve pain in the leg, (sciatic) can be caused by several factors: obvious disc pathology, stenosis of central canal, stenosis of lateral recess or at the IVF L5/S1. However, another causation especially if skiing is the only thing that exacerbates it, could be increased inflammation at the disc site or the facet joint L5/S1.
So back to questions: what have you done thus far? PT? Reconditioning the spine, hips and LE? Conservative treatment? Anti inflammatory med? Boost therapy, prednisone? Injections at the site?
Spondylolisthesis is a complication of this problem, not the primary. Yes, if there is subsequent surgery, it will have to be dealt with especially if the nerve dysfunction is directly coming from L5/S1.
Get multiple opinions, neurosurgeon preferably. Do your due diligence, ask questions, work hard with strength training, flexibility training.
Good luck.
Pursuant to your first question, if no complications post surgery (if you have it) you should be fine to ski.