Corypheus, Archdemons and 'effective immortality'

I think the short answer would be that they are continually revising the lore to suit the story they want to tell, so whenever a contradiction seems to appear we are told it was just imperfect knowledge on the part of our PC or the history/explanation that someone else gave them.

The main problem I had with the Corypheus reincarnation is that it was claimed that the reason he couldn't be killed was linked to his dragon. That had me questioning whether he always had that dragon because he was able to body hop at the end of Legacy. In ancient times the Wardens didn't seem to be able to kill him simply because he had some strange control over them that prevented them striking at him, which is why the best they could come up was to imprison him. So, as you say, the influence he has over the mind of the warden would mean they willingly accepted the transfer and so the mechanism was very similar to how the Archdemons jumped to the nearest darkspawn, which we were told are soulless, so they is nothing there to resist.

The problem I had with the Kieran transfer was the distance which the Old God soul had to travel in order to jump into Morrigan's womb if you left her at the city gate rather than into the Warden who was standing right next to them. Clearly it involved something other than having sex with her and fathering a child, since the Warden could already have done this at an earlier stage. I've no idea if Kieran is soulless once the OG has departed but it was probably more a case that for a time he had a twin soul which co-existed in exactly the same way that Flemeth/Mythal or Anders/Justice do. As he was still young, the spare soul could depart without it harming him.   Also with the Old God soul it would seem an automatic transfer, not something that is done deliberately or the Archdemon would stop itself from jumping to the warden and aim for the nearest darkspawn instead.

Going back to Flemeth/Mythal she was able to survive being killed by the warden through just a small piece of her being placed in an amulet. So this could be what was going on with Corypheus and his Dragon, except in that case surely his soul should have jumped to the dragon? This is why I formed the theory that the Old God Dragons were the split soul of either the Evanuris or the Forgotten Ones and the reason Solas was so horrified at the prospect of killing them is that it would free the soul to return to the Fade or the original being wherever it might be and the wardens were wrong about the soul being destroyed. Except in Corypheus' case making his soul whole again was what made him killable. Then in World of Thedas 2 it is confirmed that we didn't kill Corypheus at all but simply forced him into the Fade, where because he was now beset with doubt he would be destroyed because "nothing that knows doubt can survive in the Fade".   

So your guess is as good as mine what Solas really means when he says that Corypheus had learned the secret of effective immortality as it could be any one of a number of processes.

Here's another set of questions for you. Why didn't the Blight affect the Magisters in the same way it does other mortals? Most people either die or get turned into ghouls which gradually decays their mind. On the whole Corypheus was pretty lucid. He claims that they allowed the darkness to permeate their being, so deliberately turned themselves into darkspawn, yet they retained their freedom of thought. Darkspawn were drawn to Corypheus in DA2 in a similar way to the arch-demon (they rather glossed over this in DAI never explaining that is why we had a rash of darkspawn in various locations), yet he would appear not to have wanted to use his power over them. They could easily have given him his army instead of the far harder to control demons. Perhaps he didn't want them contaminating his new Imperium but then why use red lyrium? He could also presumably know where the last two Old Gods were because he should have been able to hear their song even if he could resist it.   Actually, now I think about it, why didn't the magnified False Calling have darkspawn pouring onto the surface?   The Grey Warden calling is only them being no longer able to resist the call of the arch-demons in the Deep Roads but if this was being overridden by the False Calling that should have affected the darkspawn too.

Why did the Architect's ritual with Grey Warden blood block the song and free the minds of his fellow darkspawn? Where were the Magisters during the First Blight? The Wardens only became aware of them after it was ended. Yet Dumat's corpse attracted Corypheus and was used to trap him. So what was going on there? The annoying part is that now Corypheus and the Architect seem to have been removed from play, I doubt we are ever going to get answer to some of these questions.

However, I live in hope that those last 2 Magisters will turn up somewhere along the way, particularly as Razikale was the patron of Minrathous and the kingdom of Tevinter before the Imperium, so her dragon is likely to be sleeping somewhere up there.   Also a long shot, may be they are connected with the Executors.

Did he? It is so long since I played it that I don't recall that. There was a shrine to Dumat in the prison complex and we were attacked if we desecrated it but I think that was explained as being spirits/demons that had identified with the people who had worshipped there. Still it was odd that he should have been imprisoned in a place where the worship of Dumat was carried out, unless the Wardens thought that would make it easier to entrap him because he kept returning there or something. Still, if it was an ancient shrine to Dumat then it is entirely possible that a faith spirit aided him. After all the spirit is attracted to strong faith and may have lingered there even after the worship of Dumat died out because that was a place where faith had once been strong. So if he still believed in his god after all that time, the spirit would have identified strongly with him and responded to his cry for aid.