Have you considered treatments such as spinal decompression? I was skeptical at first but after the treatments and physical therapy that went with it my back has improved dramatically.
As for working on the lathe I would probably consider getting a padded chair with castors that would move easily, something tall enough but that won't get in your way with arms and such. You will probably have to sweep up the shavings more frequently depending on what you are working on, but I see no reason you can't do at least some turning while sitting and just stand when you need to perhaps.
I want to die quietly in my sleep like my grandfather, not screaming in terror like the passengers of his car.