I would actually be fine with the Architect coming back now that we know he's probably one of the original Magisters, and therefore could plausibly have the same powers as Corypheus. But then, Leliana coming back doesn't bother me either. (Given that she can only be 'killed' in the room with magical healing ashes ...)
Yet, the Ashes are corrupted, and since you grab a pinch for the Arl first before corrupting them, it is implied the Ahses lose their healing abilities. It could have been the Lyrium, but really, you'd think the Dwarves or Tranquil would have discovered some healing properties of Lyrium, and elfroot is the go to healing substance, not lyrium. Unfortunately, even if Leliana's resurrection could make sense, it feels a little "hand of God" style writing, even if it is supposed to possibly be divine intervention, simply because there is little to no evidence to establish a non-divine hypothesis for her healing. If it is the Ashes: how do they work once corrupted? If it is the Lyrium: how have we absolutely no other examples of Lyrium healing?
Anyway. I'm kind of bummed the Architect was not in Here Lies the Abyss, now. I felt that that was one of the best segments for lore developments concerning Corypheus and the Magisters and the Black City, but it was focused on some weird Demon thing and explaining that whole aspect of Corypheus' plan. It became a bit of a diversion that focused on Nightmare as opposed to further developing Cory. If it was the Architect, I feel like more revelations applicable to he and Cory would have been explored, and the weight of Cory in the game by the end might have been more intimidating or at least satisfying.