Piriformis syndrome is difficult to diagnose. The reason why muscle relaxants aren't working might be due to the fact that it is not the right therapy or maybe not the correct diagnosis.
From what you refer there is too much going on with your left leg to blame it on the Piriformis. Moreover you don't refer of pseudo-sciatica pain down the back of the leg which should be a sign of the piriformis affecting the nerve.
Do you have other symptoms? anything abnormal in the blood tests?
I had a problem of pseudo-sciatica which wasn't due to the piriformis. the EMG was negative it only revealed a problem in the tibial muscle on the left leg which I think is the muscle that lifts the foot.
I had a systemic neurological problem of unknown origin that affected the spinal cord and a small spinal disc herniation between D6-D7. you should check the whole spine and since you're there do MRI brain/cervical/dorsal.
I have pain down side and back of leg. I have had two surgeries on my neck C6-7. I do have degenerative disk disease. I have had MRI on cervical,thoracic and lumbar spine. I have disc protusion at C-7-T1, T-7(I think forgot which one in thoracic spine) and L4-5. Brain MRI show have three lesions in the right frontal horn and juxacortical area. I had borderline positive ANA. I have been worked up for lupus and MS which all were negative. I have Vitamin D Deficiency.
It does sound like for you too there's a systemic problem that is aggravating or even triggering orthopedic problems.
I had pain in both buttocks down to the calves like diffuse sciatica-like pain not intense, not well localized just fire in the back of the legs. do you have pain in one leg or both?