I have a student who is moderately knock-kneed and has trouble closing in fifth position without bending her legs. She has a swayback, and her turnout isn't very strong. Do you have any advice?
As a teacher, knock knees can be particularly challenging to work with, because they stem from a structural condition that won't change. Further, all of the concerns you've raised can be connected back to this central issue. Because of her knock knees, you've observed that your student releases her knees, which shifts the pelvis forward into a lordotic (swayback) posture.
If she's able to stand with her pelvis more upright in third position, then have her work there as she develops more strength in the turnout muscles. Encourage her to do extra hip-flexor stretching—especially for the iliopsoas muscle—and teach her to self-correct her alignment by monitoring her feet and making sure the weight is evenly placed between the pad of the big toe, little toe and heel. This will prevent her from rolling in on her feet, which tends to go hand in hand with a knock-kneed alignment.