Thanks so much for your question, and I hope that I can help you out a little.
First of all, I need to ask why you would rule out a chronically inflamed Psoas, as in fact, all of the things that you describe indicate that this might be the case, especially in regard to the tucked under pelvis, and reversed spinal curves. I have actually written another post that explains this in detail about Chronic Hip and Back Pain
One of the biggest issues I note with any student with hip issues is the tendency to this type of tucked posture. It often develops as they are trying to engage their deep abdominals and hold turnout, however this posture actually contributes to destabilising the deep little low back muscles (Multifidus) making the true core more unstable. If a dancer maintains a flexed lumbar spine, the Psoas muscle (that attaches to the front of the spine) is often recruited to stabilise the low back. This then makes it less available for what a dancer really needs it for which is supporting the lifted leg in extensions.
If the Psoas is unavailable, then the load of the weight of the leg is transferred to the Rectus Femoris and Sartorius muscles, so this may well be the issue that she is having. Both have an attachment point at the front of the hip and the knee, so it depends on the location of the pain as to which one is being overloaded. Once correct alignment and stabilisation of the low back is achieved this will allow the Psoas to release it's stabilizing role, and be used more effectively for lifting the leg. The video below explains this in detail.