I respectfully disagree. and I cna answer your question about mumm-ra. because by the time of the original series Mumm-Ra had already taken control of the planet. He was the most powerful force on the planet and there was NOBODY there to challenge him. Since he'd taken out ANY rival to his power in the past he didn't feel the need to be active anymore. Did you see any of the people living on third earth even THINK about challenging him? No. Because there wasn't any needs. Mumm-Ra already controlled the planet and that being said he figured nobody around, including Mumm-Rana, could defeat him. So he stayed in his sarcophagus and everyone else just did what they wanted.