What can sciatica (when other causes are ruled out by pcp, rheum, & physiatrist, including w/ use of mri & ultrasound) be caused by? lupus?if so, how?

Many possibilities: If lumbar spine MRI is normal it's pretty unlikely you have a pinched nerve in the back. Some other possibilities include nerve irritation in the pelvis. Look up piriformis syndrome and see if that matches your pain. Also, the sacroiliac joint can sometimes cause sciatica like pain. Sometimes this is muscle pain. You should get a second opinion from non-surgical spine specialist.

Spine Pain Options: This pain in the distribution as you suggested is the result of an irritated nerve or facet joints or other injury typically in the lumbar spine (low back) which are caused by herniated disks, spinal stenosis or degenerative disc disease, etc requiring further evaluation by a spine specialist and may be candidate for facet injections/radiofrequency ablation and epidural steroid injection.