Hi Pat ... thanks for asking this question in the forum. Perhaps other Yinsters who have a similar condition can share their experience with us.
Firstly, you are right - I am not a doctor and can't give you any medical advise, especially without having seen you in person. I can offer my thoughts however, and leave you to use your inner guru, and your health care provider, to work out what to do.
I know that to "stenose" means to narrow, and so spinal stenosis is a narrowing of the spinal canal causing pressure on the spinal cord. This can manifest as pain in many areas of the lower body. Unfortunately there are many possible causes of the narrowing of the canal. From your description it sounds like the cause was some sort of one-time trauma, not a congenital deterioration. Again, without knowing the exact cause, it is hard to work out the best remediation for the condition.
You are really asking one simple question: are passive yin extensions okay if there is no pain? In spinal extension we are creating compression in the vertebra: normally that is a good thing. Compressing the bones helps to stimulate the osteoblasts to get busy and build thicker bones for us. Since once possible cause of spinal stenosis is osteoporosis, this should be good. However, if the narrowing of the canal is made worse by compressing the disks, this may be bad. The real key is - is there pain, both at the time you do the pose and after your practice?
There exists a lot of evidence that passive extension of the spine promotes healing for many back problems. However, there are always cases where it doesn't help and may make the situation worse. I would suggest, with your health care professional's guidance, you do try the Yin Yoga postures and really check in on how you feel during and after each practice.
Start with the easy backbends like Sphinx. In time, more to the higher positions (ie: resting your elbows on bolsters or books). Before trying Seal, which is the deepest backbend, you may want to experiment with Saddle pose, providing your knees and ankles approve. [You can find all these poses described in detail in my book YinSights, right here in the Asana section of this site:
http://www.yinyoga.com/ys2_2.0_yinyoga_asanas.php]
Let us know how this works for you. I am sure that there are many people who would love to learn from you.
cheers
Bernie