Q : Are all physical or mental disorders considered "medically-related grounds of ineligibility"?

No. The civil surgeon must determine whether there is harmful behavior associated with the disorder in question. If you have a history of a physical or mental disorder, there must be a harmful behavior that is likely to recur in order for it to make you ineligible. If your condition has no associated harmful behavior, then you can proceed with your application process and no waiver application is necessary.