Doing any extension exercise would be mad in my book there is clearly no mechanical advantage to this in a patient with stenosis. You need to make the foramina gap (flexion) not close (extension). Remember why this condition happened the disc bulged, then losest height, then caused rubbing of the facets; which went through a defensive osteophytic formation to stabilise.
By entering the facets you will make them worse!!! Before you make them better. By using flexion distraction method you should see results by forth treatment. This can also be by hunching them over a pillowed stool and genteelly pushing cephalad aspect of the spinous processes individually into flexion.
Also get them to sleep in fetal position at night. Assuming that offers relief - does it?
One thing of note from your discription: is pain when they bend forward is it when sitting, standing that cause pain?
Is it due to muscle spasm. Very likely to happen in a failed spine like this.
Or is it due to facet capsule aggravation very common to happen in grade 1 spondy.
Or is it just nerve tension.
Grade one spondylolisthesis is stable if that helps.