The Archmage? Is this some black metal band?
Go to Candlekeep forums and see what some fans think about that.
There's already a 'separate' Greenwood Realms canon, published as 'Ed Greenwood Presents Elminster's Forgotten Realms' in October of 2012.
With that said, having Greenwood publish Realms material on the DMs Guild is pretty much a no-brainer.
If they disagree, they're wrong. Forgotten Realms is a sandbox for the world's greatest storytelling game. The stories you tell cannot be superseded by any other source including official ones. If I decide that in my story, Icewind Dale is full of dinosaurs and Chult is full of yetis, Candlekeep cannot override me. For my players, it becomes the primary canon.
I think that there is enough reverence for Greenwood that anything he produces will be considered canon. Even if it deviates from published materials, I think WotC will do their best to fit it in or explain the discrepancies away. There might even be an agreement to physically publish the material if sales hit a certain point.