I think, ultimately, it won't matter whether the Architect was spared or who Corypheus possessed, because the rogue wardens are trying to awaken an Old God themselves. Fiona was questioned extensively about her experience with the Architect and cure, as well as the plan to immunize Thedas with taint. The wardens might have reached the same conclusion that the Architect did - by feeding an Old God warden blood its calling might end. The maps Anders provided must have been in existence very shortly after the end of the blight, and were probably much older. Nathaniel seemed to favor the Architect's plan and might be a natural choice for a leader, and the wardens had brought enough explosives for mining. I had thought the wardens were going into the red lyrium trade as a retirement plan with no blights around, but what if there was an Old God under Kirkwall?
Avernus, the Architect, Corypheus, and Isseya showed how multiple individuals can come up with different ideas on how to use the taint to their advantage. The Architect manipulated Remille and Genevieve in the Fade. Corypheus was able to switch bodies, like Flemeth. Since there was more than one awakened darkspawn, what would happen if there were other ancient magisters running around? One of them could mind control wardens the same way the Architect did.