I fear you are on an impossible course with this young woman I have had a few of these in my time (I have just retired) Unfortunately They have usually turned out to be complex anxiety problems and sadly a form of attention seeking. In my experience they do get better eventually but very stressful for the therapist as well as the patient. If the knee moves fully under anaesthetic and no abnormality has been found with extensive investigations it is reasonable to assume that the problem lies deep in the mind. I would try some deep relaxation techniques and some gentle passive flexion. Enormous amounts of encouragement will be necessary

Good luck