So what's the distinction betwee "Master Quest" and "Hero Mode" in the Zelda series? Master Quest is mirrored, harder, and with reworked dungeons, while Hero Mode is just harder?
Yes. Master Quest was the result of Ura Zelda. Ura Zelda was basically an expansion pack on OoT but then Eiji Aonuma said he didn't want to redo OoT dungeons because he thought it was lame and because he was the dungeon design lead. (Meaning he had to do most of the work.)
After complaining a bit, Aonuma decided making a new 64 Zelda would be better. Miyamoto agreed but said Aonuma would have to make the game in under a year. And that's how Majora's Mask was made. Those crazy mofos.