Amending my musing into a theory:
We see with the Elvhen deities that they have the ability to shift into different bodies - with consent (Dragon Age Origins - Flemmeth correcting Morrigan's misunderstanding).
What if the Old Gods had the same abilities (were possibly Elvhen deities on the otherside of the civil war that destroyed the elves - but that's a different theory). They then took on priests and taught them blood magic (the first Tev. Magister).
At this point in time - perhaps they also started taking on their forms and giving the High Priests the ability to soul shift (logically this would mean that the God would be dealing with the same person and not have to reteach them etc over the years [it's also another plus in the former elves used to immortality column] and training specific bloodlines - a big past time in Tev.)
When the 7 assaulted the Fade, they came back blighted and were the first darkspawn. In his lair, Corypheus speaks of the unbearable silence and calling out for Dumat - he also does this when he wakes from his slumber in Legacy.
If we assume that the first thing the blighted 7 did is call out for their Gods, perhaps this instinct was instilled into the Darkspawn - which is why they desperately dig looking for the Gods, not stopping until they hear their song and can get to them.
Now, once blighted - the Old God / Arch Demon has the ability to shift into any of the Darkspawn or tainted beings and survive - with the exception of the Grey Wardens.
The magisters like Corypheus can shift into any tainted being - no exception.
What is the difference here?
The joining ritual of the Grey Wardens is blood magic - it uses a drop of arch demon blood.... blood magic that ties directly to the Old God, and possibly still contains their power - which would put the Warden on a level playing field so to speak, which allows the soul to be nulified.
Or, is it a size and power difference?
If we assume that the arch demon blood in the joining chalice acts like a lightning rod for the soul of the arch demon, then what if we consider that Corypheus can enter a Warden's body because he has a soul that was formally human and powers that either come from his blighted state or were gifted by his God.
The archdemon on the other hand is attempting to squeeze an entire soul in on top of another one (not the consensual coexistence of the Elvhen Gods) which because there is not enough space or life energy, obliterates them both.
When it comes to Corypheus and his dragon - he creates it out of hubris and instills a part of himself in it. We're told that his ability to soul shift is disrupted by the death of the dragon - perhaps this is because he's being whacked with a wallop of his own power and like a computer, it temporarily scrambles his systems - giving the Inquisitor the opening they need to kill him (I think... because if they sent him back into the Fade like they quipped, then that's a really REALLY stupid thing to do).
Sorry for the essay.... my thoughts kind of got away from me!