unable to sit

[neurospast]

I am not sure where exactly you have this 'buldge'. in case it is at the top of the crest of the pelvis (crista iliaca) it could well be an inflamated ligament like ilio-lumbar ligament which holds L5 into possition (with other ligaments). it would feel like a spongy gel type of substance which is painful from achy to sharp and could cause via via some referred pain. Look into the local anathomy. a good atlas (to my opinion) is sobota because it is more like drawings instead of cadaver pictures. The local anathomy with knowledge of biomechanics could help you on your way.
I am still surprised that you did not find any triggerpoints because gluteal muscles, iliopsoas, quadratus lumborum and other muscles tend to become hyperactive when the back becomes unstable (which puts a strain on core stability)

[morwenna]

Not a spinal expert either,. ..... but after all these months is it not possible/likely that she has some adverse neural tension/tethering of the sciatic nerve?

ANT stretches and mobilising techniques might help?