On why it has to be Zenos, I have a feeling the circumstances behind his birth is going to be very important for this. Zenos was considered an experiment. An experiment in what? An artificial Ascian? Recreating a lost soul? Or something else? Whatever it is, that has to tie into it. He outright mentions he has remembered the fall of Amaurot since he was a child, yet he did not have the Echo. That feels like it too could be important. Was that memory implanted into him? Or was he truly there, albeit as something else?

Wild Mass Guessing, he may have been created as a way to contain and study The Sound, and if so, that would explain why it has to be him, he is the original and current threat to the planet. Such a thing sounds incredibly crazy, but remember, one of the Unsundered Ascians was a mad scientist.