Passive stabilisation is key here. The back has failed! It is really pain management until pain can be significantly reduced. I am a chiropractor so we pick up and treat a great deal of these cases - here is my protocol for spinal stenosis from stabilisation phase of discal degeneration:
Passive support a brace is key here bauerfeind a German company do great ones fitted for spondylolisthesis.
Next flexion is key the osteophytic formation around the facets is impinging on the neural forama press in on the nerve - leading to pain down legs. We have a device called a flexion distraction bench. This allows the pt to lie prone and for us to passively induce flexion by lowering legs. If you don't have this it's a problem because the spine is so unstable that if you passive do this by hand it would be hard to control and even hard to replicate. However if pt was placed in swimming pool so weight on spine is reduced you may be able to either flex hips to offer some natural flexion traction into lumbar segments.
Next use EMS to stimulate multifidus muscles in a aim to enhance natural stability.
Do this 2 week for 2 weeks then 1 a week for 3 and review pain levels.
Note: you concern about flexion further damaging the discs is unwarranted in a pt of this age. Would be good to review some of the latest research on disc anatomy and aging. Basically the disc has at this point losses most of its water content and is unable to herniated further.
Does that help?