The one thing that keeps me from believing that the Sand Superman saga (either version) is the key to all of this is that I don't know if DC is willing to uncanonize the Death and Return saga. I mean it survived Zero Hour, Infinite Crisis and New 52 ( sorts) . Is DC willing to say the Superman who was killed by Doomsday and later resurrected is some fake? I mean, I could see them leaving the rest of the last 30 or 40 years of stories on the floor, but I don't think they are willing to sacrifice the sacred cows of Post Crisis Superman stories, especially since THE architect of that story is involved with this whole Rebirth thing.
Unless the powers that Be retcon the Sand saga occurring post Death and Return. If they do, then all bets are off.
I'm still thinking the ElectroSupes Red and Blue saga is the key or one of the keys to all this, but perhaps it's all red herrings and the final answer is something completely unexpected. Much like the Spanish Inquisition.