*dons tin foil hat*
Theory One:
Archdemons aren't corrupted by normal darkspawn. They can only be corrupted by their corresponding High Priest. Either that, or the High Priests were the only ones outside of the Wardens to know where, exactly, in the deep dark their own gods were buried.
The darkspawn dig in the vague direction of the call, and the remaining Magisters take control of large groups and direct them to the real target. This was mostly inspired by the fact that the Architect was once Urthemiel's High Priest, and he was the one to corrupt him in the end. I enjoy the poetry of it.
Theory Two:
The Old God beneath the Western Approach is still there, and is probably Razikale.
While I don't know many spoilers about Jaws of Haakon,
Then there's the business with the Weisshaupt Wardens, the First Warden in
Last Flight investigating blood magic and subversion within Warden ranks going back centuries, and so on and so forth. This all feels like the foreshadowing to Razikale making a big entrance in a coming up DA game, which I'm all in favor for. Especially if the corresponding Magister, the Augur of Mystery, shows up.
As for Razikale or Lusacan being beneath the Western Approach: The Wardens have many outposts throughout the south. While demon-summoning and blood sacrifice is shady enough for them to retreat to their most isolated and abandoned outpost, the urgency of their Calling coming means I'd doubt they'd want to go very far from their first target. They've set up in the Western Approach for privacy, to build their auxiliary demon forces, and then to head a short distance to crack open Razikale's prison. I've heard reports of people seeing a flash of something draconic and hearing strange sounds from the collapsed mine, which is deliciously creepy.