Are there any other theories as to how endometriosis is developed besides this backflow theory?

Yes: It may be spread through blood or lymphatic flow. It may be the that surface lining of many organs can transform into endometriosis. There is likely an immune system mechanism as well. No theory has ever been proven. In fact some women who have never had a menstrual period before have endometriosis.

Metaplasia: The most likely theory for how endo forms is that of peritoneal metaplasia. Cells in the lining of the pelvis can have the potential to turn into endo under the influence of estrogen. This explains endo in the lung, brain, and other sites far away from the uterus. Endo has been found in babies who have never had a period, and also in men.