I'm troubled by an adolescent specialist who never heard of serious cramping in a teenager. Is this a specialist in general adolescent medicine, or a gynecologist? If not a gyno, I'd see one.
I agree with Ms. May that a different type of birth control pill may be indicated. I agree with you that heavy duty Naproxen isn't solving the problem - it's just treating the immediate symptom but could be causing so many other side effects especially on the digestive system. She's already having stomach problems, and there is a lot of evidence of intestinal issues too.
I would think it's less likely that someone this age has fibroids or major PCOS but anything is possible. I'm hoping the ultrasound will give you some information. If you do decide to see another physician, remember that you (or your insurance) are paying for these tests and the results can be taken with you to see any other physician.
While your daughter definitely knows her body, it's possible that inflammation in the gastrointestinal area is putting extra pressure on her uterus, so while it's not entirely gastro, there could be some involvement there that's making her problem even worse.
Have you looked into any over-the-counter anti-inflammatory food formulas? That's worked so well for women in my professional circle and their daughters (teen issues through menopause). We attend regular food science trainings and information sessions, and there's a tremendous network of success stories including physicians, pharmacists and nurses, plus food scientists. There's a combination of something that helps with menstrual/hormonal issues as well as inflammation. That's another route to go that won't interfere with anything medical that you do.