To be fair as well, after that scene Aymeric does tell you that he thinks he expressed himself poorly. Perhaps he was kicking himself afterwards, and thought he could have made a better argument.

On what is believed now: I suppose my point is that it doesn't make sense, to me, for there to have been a single lie produced by a cabal of deceivers. Rather, we are probably dealing with a growing tissue of falsehoods. At the time of Thordan's death, perhaps all the founders of the high houses said was that the dragons turned upon them and slew their beloved king. They omitted, but didn't lie directly. Over the coming centuries, the details of Thordan's death passed into legend and were exaggerated. I don't know where the story of the elezen first coming to Coerthas came from; perhaps that was bundled into the Thordan story centuries later. To bring us back to the topic, this question about the textual history of the Enchiridion is fascinating.

On dragon memories, I am sure that they remember some things extremely clear, but as you say, their psychology is not like human (or elezen, hyur, etc.) psychology. Their history is part of their present. There's something Hraesvelgr says to Estinien atop Zenith that springs to mind. Estinien asked Hraesvelgr why he should believe Hraesvelgr's version of history over the church's, and Hraesvelgr answered, "What thou choosest to believe is immaterial. The betrayal that yet haunts mine every waking moment is no less than the truth to my kind. And Nidhogg meaneth for Thordan's people to suffer for this sin till the end of days."

That makes an interesting point about truth, I think. Nidhogg is not driven by a historian's truth, so to speak. He is driven by a personal and subjective impression of Ratatoskr's death, that he constantly relives. The impression of how it felt to know that his brood-sister was murdered by treacherous elezen is a permanent brand on his spirit. That's not something that can be changed. If, perhaps, someone were to find a reasonable historical argument that cast Thordan's actions in a new light, or even justified them, that would do nothing to alter this vengeful memory. Or neither is there any gift, apology, or recompense that could be offered that would alter the memory itself. Nidhogg is haunted by the sense of how it felt at that exact moment in history. The memory is eternal.

I find that much dragon behaviour makes more sense if their strongest memories are eternal in this way. It especially contextualises their tendency to brood. When you first meet Hraesvelgr, he is a broken dragon, sorrowfully reliving both his memories of Shiva and his memory of Ratatoskr's death. Or we can consider Tiamat, who is so consumed by the shameful memory of what she did to Bahamut that she is prepared to sit motionless in an Allagan prison for eternity. I would not not be surprised if the oldest dragons usually do end up consumed by memory. They are monomaniacal beings, whether Hraesvelgr for lost love, Tiamat for guilt, or Nidhogg for rage.

Anyway, so I don't think the elder wyrms lie to you, and neither do I think that their memories are false as such. But I think that their memories are of limited usefulness when it comes to discovering exactly what happened.