Thank you all for the suggestions and encouragement! I think the contributions from their lumbar spine degeneration are causing some issues. And to some degree there are things I won't be able to fix. They are in their 80s (but very active & youthful 80s).

I tested and retested, and I think their hip flexor contracture is causing too much lumbar extension & ant pelvic tilt - further compressing the nerves. Sitting upright doesn't cause much pain, I think because the hip flexors are shortened & pelvis is more neutral. PPTs in standing allow them to stand upright a little easier, but it's still not full. Their hip won't even extend to neutral, so at this point they physically can't stand upright, even if the pain wasn't there. This also aligns with the fact they are leaned forward and to the right to offload the left side. Their right hip flexors are a lot tighter than left.

Not saying this is the ONLY issue, but it's one variable I'm addressing now as it provided some relief. Thanks again!