7 is the only one that can be properly answered. Kind of.

Whether or not the moon exodus plan was really viable depends on whether or not Meteion's song of oblivion had to be directed. If so, escaping Etheirys to settle a distant, random planet she would need an unforeseen amount of time to find would work. If not, nope. We don't know one way or another though.

The moon wasn't fully finished because making it into an ark was retrofitting; originally it was nothing but Zodiark's prison, until thousands of years later Hydaelyn created the loporrits and charged them with making the moon inhabitable by the people of Etheriys. Besides which the moon is inhabitable, it's just not quite... to scale because the loporrits didn't know the modern people of Etheriys are a lot shorter than the Ancients. It doesn't have the creature comforts people are accustomed to because the loporrits got all their knowledge on modern Etheriys from the infamously austere Sharlayans, that's all. The main obstacle to the exodus is that there's no quick transit between the moon and the planet (other than the Tower of Babil, which wasn't completed until very recently and is dangerous to access besides).

The exodus was also the Plan B, to be executed only if the people of Etheriys wanted to flee rather than fight. Confronting pain and sorrow rather than running from it was kind of a major theme in Endwalker though, so...