The Magisters Sidereal in Dreadwolf

The problem with this is that it is a fixed identity and we would be infected with the Blight. Unless we had already done one soul jump but even then it would have to be a body with the taint, so it would gradually succumb to it. Kieran may have seemed okay but he was only 10 years old when we met him and it was only a small amount of taint to begin with, so it would have taken longer for it to spread. Besides he was the vessel of an actual Old God, not just a tainted Magister.

As for our identity, even with the amnesia, for the Magister Sidereal to have any relevance, eventually we would have to remember. It would be like the Inquisitor on steroids because instead of forgetting just one small section of our life, we would have to forget everything. After all, from what I recall, the Architect did not have any particular powers other than being an intelligent darkspawn that could resist the call of the Archdemon and found a way to allow others to do the same. I think for our PC to be one of them would just be too problematic.

However, that doesn't rule out one of our companions being a Magister Sidereal in disguise. Now that would be intriguing. They probably could remember but kept quiet about it. As you suggest, it could be the First Warden or perhaps some lesser Warden that no one would suspect of being anything different. So, they would have their own agenda, probably linked to awakening their Old God, either Razikale or Lusacan. I did have the theory that perhaps what is really required to wake the Old God is their respective Magister. In other words, it wasn't just coincidence or lucky chance that the Architect was the one to wake Urthemiel. Nor would that automatically make them evil, particularly if it didn't automatically result in corruption of the Old God into an Archdemon, although to be honest I think they are already corrupted in their prison before the darkspawn break through and that is why both darkspawn and Grey Wardens can hear their song, because it is a corrupted song. The Magister Sidereal might well be opposed to Solas, particularly if they had learned that he had originally been the one who imprisoned their god.

I think Razikale may actually be underneath Minrathous. In her short story, Calpernia could sense something powerful in the depths of the city, although that might have been hinting at the entity that was revealed in the Streets of Minrathous in Tevinter Nights.

Alternatively, Minrathous is a remnant of Arlathan when the rest was destroyed and was originally one of the cities of Mythal. The reason Andraste was being encouraged to go to Minrathous and her crusade would only be finished when the city was freed of the Magisters, was that Minrathous was the key to defeating the other Evanuris, as embodied in the Old Gods. I definitely think that Minrathous was built over the ruins of an elven city. However, it is actually on an island jutting out into the sea. Mythal was said to have arisen out of the sea in the old Dalish legend, which might point to her originally being some sort of water spirit, which would fit with her temple in the Arbor Wilds being in the middle of water and the Well of Sorrows. The being that rose up to confront Corypheus definitely looked like a water spirit.

However, the legend could equally be referring to the waters of the Fade. The Chant of Light has the following:
Here lies the abyss, the well of all souls. From these emerald waters doth life begin anew.

I did find it odd though that in Trespasser, Maryden sings a song Mercy for the Elves, in which there is a line: "Arlathan fell, deep beneath the ocean floor". That is strange because the Dalish legend has it sinking beneath the earth but that was only the part of it in Arlathan Forest and that was dedicated to Andruil (see Tevinter Nights). So, perhaps as the great city at the centre of elven civilisation disintegrated, the part dedicated to each of the gods, sank into a medium that corresponded with the element they are associated with. Dirthamen was associated with secrets and dreams, so his bit ended up in the Fade, whilst Falon'Din was actually connected with the secrets of the Void, so his bit is there. Elgar'nan was associated with lightning and the Sun, so his bit was left suspended somewhere, perhaps in the Crossroads. Sylaise was associated with fire, so her part ended up in the depths of a volcano or in the Deep Roads lava flows. Mythal's sank beneath the sea but a small portion was left as Minrathous.

That doesn't account for June or Ghilan'nain but since June is associated with crafting, maybe his legacy is to be found in all of the others, whilst Ghilan'nain created monsters and was not one of the original Evanuris credited with forming Arlathan.