Based on what Bioware said about their concept for DA:I, they had split the story in two. What note does Trespasser end on?
Corypheus is what he was written to be: the decoy antagonist. It's just another deconstruction.
If deconstruction's the intention, then the writers misunderstood what a decoy antagonist is. Therefore, the deconstruction doesn't work.
A decoy antagonist would be a front for the true conflict of the story. Essentially a big bad whose revealed to be a misnomer for the true villain of the story whose really in charge. Something akin to how Maul and Dooku are the decoys to mask Palpatine's behind-the-scene manipulations. Maul and Dooku are just public figureheads and killing them doesn't stop Palpatine whose the true threat in the Central Conflict.
The description of Cory as a decoy antagonist doesn't work even in light of Solas' revelation because Cory's actions and motivations were independent of Solas. Solas had his own plan, but he wasn't controlling Corypheus at every turn, he enabled Cory to accelerate the magister's plan based on a miscalculation and failure to ensure that Cory would die when he was supposed to.
Solas and Cory had independent intentions that happened to be tied to Solas' foci. Not to mention that Cory and his actions are the clear driving force behind the central conflict of Inquisition. Cory's the one who manipulates the Grey Wardens; plots to kill Celene and destablize Orlais; and corrupt the mages/templars into his personal armies. Solas had nothing to do with any of that, though he certainly deserves credit for enabling Corypheus to cause the Breach.
Stopping Corypheus is actually what enables Solas to continue with his own plans or else Cory would succeed with his goals or destroy the world before the Dread Wolf would have a shot. In this way, Solas would count as the antagonist of a different conflict that would be loosely related, but largely independent of Inquisition's main conflict.
To compare to a better example of a decoy antagonist, my first thought goes to Saren from ME1. It's initially believed that Saren is taking advantage of the geth and reaper technology to find a hidden weapon that could destroy the galaxy. In truth, Saren has been indoctrinated by the Reaper Sovereign whose trying to find a backdoor into the Citadel to kickstart a mass genocide by summoning more reapers.
Another example would be General Scales from Star Fox Adventures, whose actually just a puppet for Andross. Or Jihl from Final Fantasy XIII whose just a front for the Fal'Cie leader.
In short, Corypheus doesn't count as a decoy antagonist. If the developers want to insist that he's a decoy, then they're just short-changing him even more which is a shame.
Corypheus deserved better.