These dialogues don't exist Morrigan know nothing of the veil created by Solas.
In the chant it is never stated that the Maker created the veil but that he created a whole new mortal plane for mortals.
Sorry, it wasn't Morrigan. It was Yavana, and it was regarding dragons in general, but most likely she was referring to the Great Dragons, which the old gods are.
"Your heart beats with the old blood, as well. Where do you think it comes from? It sings of a time when dragons ruled the skies. A time before the Veil, before the mysteries were forgotten. Can you hear it?"
As for the Chant, I will have to look through it again. Perhaps the idea that the Maker created the Veil is stated in separate codices.
Grey Wardens only think the soul has been cancelled by the Ultimate Sacrifice. They don't know for sure what exactly happens; simply that both the archdemon and the Grey Warden involved die. How do they know for sure that the spirit/soul of the dragon god doesn't simply return to the Fade after passing through the Warden? The death of the Warden could simply be from the shock of the spirit passing through their body or the wave of energy from the passing of the archdemon.
That may be why they believe in soul cancellation, but the Dark Ritual proves that the old god soul flowing through the Warden and the explosion are not enough to kill a Warden. Based on that evidence, it is indeed the soul of the old god clashing with the Warden's.
It is a logical conclusion. The soul of the old god would not go to the Fade. It would go to another darkspawn. The only way that transfer is stopped is if the soul is destroyed. The Dark Ritual merely redirects the transfer to a different target.
Back during the 1st Blight there were several occasions where Dumat was killed but not by a Warden and then the Wardens observed him resurrect into another darkspawn body. They then had the theory that if the Warden struck the killing blow, may be the soul would jump to them. In the final battle with Dumat, the archdemon was slain and didn't jump to the nearest darkspawn. However, all the Wardens engaged in that battle were killed, so there is no way of telling who actually struck the blow and whether that is actually what stopped him soul jumping.
Sure it is. He resurrected before, but now with their new strategy he didn't. Therefore it must have been the strategy that worked.
For example, if someone had done a dark ritual before the battle, the soul could have gone elsewhere and no one would be any the wiser since there were no suspicious survivors, as there were after the 5th Blight.
But this wasn't the only blight. There were three more after that in which the Wardens observed a single Warden striking the final blow. This is the reason why Riordan is able to discuss it with you. Also, the Dark Ritual couldn't have been created until after first observing that a Grey Warden could destroy an archdemon, since a Grey Warden is one of the ritual's "ingredients".
So from what I can gather, the Wardens had a theory that seemed to work and thus they assumed it worked in the way they had reasoned it would. Unless someone with expertise on the destination of souls/spirits after death was able to confirm categorically that the Old God spirit hadn't returned to the Fade, how would you know for certain?
They probably did have people who searched the Fade for the souls of the old gods and the fallen Warden who delivered the final blow, just to verify. They did it after Corypheus.
I certainly wouldn't go by anything that Morrigan said since she had a vested interesting in making you believe that what Riorden said was true. She may even have believed it herself, since Flemeth was the one who told her and may have concealed things from Morrigan. Morrigan can be awfully cocky about what she knows but it has been proven several times that she doesn't know as much as she thinks she does.
All Morrigan said on that issue was that the Grey Warden who strikes the final blow dies, and she was correct about that.
The OGB plot line prove that souls can't merge(they are not merged they are two souls into one body)so i doubt that souls can destroy with each other if that was the case the DR would have killed the soul of the child and he don't die even when Flemeth remove a soul,Flemeth herself had 3 souls.
It works differently in different cases. Sometimes it produces a twisted abomination. Sometimes a normal looking abomination. Sometimes one that can turn from human to demon form and back again. Sometimes the abomination remains human. And in the case of the archdemon the souls clash and obliterate each other.
The closest comparison is the twisted abomination, I believe. In this case the mage fights the demon possessing him or her, but loses, and this results in a twisted version where neither the mage or the demon exist as they once were. The difference with a Grey Warden and an archdemon is that the Warden has mastered the method the archdemon uses for possession. So it is an even conflict with no way out except for the two souls to destroy each other. In the case of a darkspawn, the archdemon simply takes over the empty shell without an resistance.
I doubt that in a US souls can be destroyed because it does not make much sense to have two different mechanics for the same elements(souls) in the same universe and with the same laws unless these writers want to follow some twisted logic in order to favourite the ritual.
But there are different circumstances involved. So of course they can be different.