Not back but neck. It may make the discogenic or degenerative/arthritic pain worse in the back but your spinal canal is opened more in a sitting position while in the neck keeping your neck upright or slightly extended keeps the stenosis unchanged or worse unless neck is tucked or flexed forward in addition to aggravating any arthritic changes..
Yes. I suppose long trips in a car due to the lack of ability to move about would make any spinal condition flare up. However, with spinal stenosis, a slight flexion in the back (such as leaning forward on the steering wheel like leaning forward on a shopping cart usually gives the spinal canal a little more room and is much more comfortable than extending the spine or staning upright. Dr. J.
Unlikely. Depends on your symptoms but more likely the period of immobility associated with being in the car results in pain once you get out of the car.