Hi
Great discussion - veery informative and knowledgeable. I agree that a spinal Wikipedia reference-linkMRI should be undertaken, including Tx & Cx - to exclude other areas of potential compression. The neural tensioning is the biggest hint - I agree that a slump or tilt in sitting could be aggravating symptoms. Thoracic spine is a likely area to look for neural compromise.

What are the erector spinae muscles like? If unevenly tensioned, a subtle apparent leg shortening can occur with a hitched pelvis. The muscle tensioning this can cause will impact pelvic sitting dynamics. Assess by using a measuring tape on bony prominences, in supine.

Try low level provocation tests to determine whether neural tethering may be at play here. Tethering can occur a considerable distance from the demonstrated symptoms, and can occur above and below eg headache and sacral pain.

Try erector spinae stretches, dural stretching etc.Trigger point dry needling is effective if muscles will not relax sufficiently - will not assist if tethering has occurred within the spinal canal.

Just a few extra thoughts to throw into the mix.Good luck - sounds like you have done welll so far, and the other posts should be helpful.
MrPhysio