Hi Pad,
These are difficult to treat, no question. My first question is how bad is the spondylolesthesis? You mention a grade 1?
In that case, while I wouldn't do end of range extension, I'd definately get them prone. If the client is getting leg ache from the stenosis this may well bring on the symptoms if they are too lordotic, but often you will find they tolerate prone, even if you have to put a small pillow under the stomach. From there you just do a very slow progression into more extension.
I agree that stabilisation exercises may help, however, if you can do some extension to reduce the disc bulge ( age may be against you here) then the stenosis will likely reduce as well (as the disc is contributing less). I have found in the 45-55 age group this works well to clear both flexion and extension, but it is slow (weeks) and they need to be very strict on avoiding flexion postures.

For the stabilisation stuff pick exercises with an extension bias.

Hope this helps.

BB