Hi there....
I don't think you mentioned any mechanism to injury, but from what you describe I still think it could very well be discogenic. However if your patient was quite acute on this assessment it's obviously harder to get a better clinical picture.
The point that you mention pain/discomfort on sitting thru R buttock could also be tight piriformis (usually spasms with back/pelvis pain anyway), this would of course impinge on the sciatic nerve, most likely explaining her leg symptoms. It might also just be compressing thru their R side aggravating the lesion
I would agree with your Mackenzie approach, however I'd also be adding prone extensions, and if symptoms centralize or reduce then you know treatment is successful. If patient generated lateral shift corrections are not successful (if you believe there is a significant shift), you may need to get stuck in yourself.
If it is disk and it does reduce and becomes more stable, then mobs may be needed to free-up that secondary stiffness. However manips to me seem to be much more effective and instantaneous in relieving joint pain
Other than this, yes! core exercises, plus reducing any sore spasmed muscles (piriformis, paraspinals, quadratus lumborum, gluts etc)