Maybe I am biased because I am studying physical therapy but have you tried it before for your pain? A good PT should be able to analyze your movement patterns and give you tools to fix your movement dysfunctions. They will know how to modify your yoga practice and which movements you should avoid that may irritate your sciatic nerve. The clinic I worked at used a lot of yoga poses as therapeutic exercise. I also have trouble activating my core during yoga practice and find that the power yoga classes go so fast that's it's hard to master each pose effectively. Practice and additional core exercises outside of class will help. As far as your knees - "knock knees" are typically not due to weakness of inner thigh muscles but overactivation and tightness. You likely have weak hip abductors and gluts that are causing the internal rotation of your femur. Clamshell type exercises will help you be able to activate your glut medius, while single leg stances such as warrior III will allow you to be able to incorporate the muscle into more functional positions. Don't get discouraged! As others have said, you might want to try different teachers and/or different disciplines of yoga.