Pobi,
Have not experienced during my lease term even though I do suffer from being older and subject to random back issues. Sorry to hear. Definitely try different positions as the seats are firm and use a prosthetic if it helps. Addressing back pain is complicated.
As to tires, your safety aside, I am told that dealers throw out old tires in any case. When I return my originals which I removed off the car, they will only have had 2000 miles put on them but will be 3 years old. I switched to softer ride all season non run flat Michelins.
Speaking of that, is it the seats or are you experiencing the hard ride that comes with a stiff bottom and run flat tires? The ride smoothed out when I switched from run flats.