What if we've been approaching this all wrong? We're trying to frame everything as if existing games are their current timelines "end point" and fitting it into that.
It's safe to say this is not on the Wind Waker timeline. Too much doesn't fit and the stuff that does fits better into other options. Which leaves us with Child or Downfall.
It may take Word of God to finally settle this, but here's just some food for thought.
Zelda uses the completed Triforce at the end of BotW.
She did not have it during the "flashback" portions of the game as the 'Goddess' would not speak to her.
We eventually learn that the one speaking to Zelda is Fi.
One of the lines Zelda tells Link when giving him the Light Bow is "Courage is never lost", implying Link may not have the Triforce right now, but it's still with him in some capacity and he had earned it's blessing at some point. (DLC explanation?)
Ganon's Final Form was taglined "Malice and Hatred Incarnate". Downfall timeline is full of botched sorcery and half completed attempts at resurrecting Ganon. That can't have a good effect on your mental well being.
My reason for pointing all of this out, is what if this takes place post Zelda II?