That's cherry picking! How can Corypheus be the only unreliable narrator? It's villain bias.
That's a failure of reading comprehension! I didn't say that Corypheus was the
only unreliable source. Get over yourself.
Sera may not directly lie but all of her omissions are lies of conscience. She lies as much as Bull about her 'friends' and will not reveal them. A lie of omission is still a lie. Cassandra does lie about the efforts of the Seekers. She admits they were doing things that directly lent to the mages rebellion by seeking out specifics instead of acting when they knew they should have. At first she madly blames Varric for lying about the whereabouts of Hawke, suggesting that was a catalyst to the conclave exploding and the Divine not having a protector or as a direct result of the happenings in Kirkwall. None of that is true. The mages rebelled from systemic abuse in the circles. Kirkwall may have been a fuse but the powers that be like the Seekers did not act. Cass regrets this. She lied to herself about the responsibility she held as the Divines acting right hand.
Again, I don't think you understand what lying actually is. If you believe that it is "failure to fully disclose every possible fact at one's disposal and be a metaphorical open book to other people in relatively casual conversation", then you have a different understanding of lying than pretty much everybody who speaks or reads the English language. Similarly, emotional overreaction to trauma and an inability to be completely logical about serious events isn't automatically a lie either.
We are told not to like Cory and disregard his memory of the Black City because it is heresy and we are the Herald of Andraste. But all through the game, event after event makes the player come to terms that all of the foundations for many of their belief systems, humans and elves alike, (dwarves too in the Descent) may be false or skewed. This is because of the things in Cory's tale are true! Many confirmed throughout the game. Like his lineage through the Wartable ages ago. Or that his accounts were also supported by chants in the World of Thedas volume 2. (According to the boards here.)
What does it mean when something is impossible to falsify when evidence supports it?
Aspects of Corypheus' story are indeed possible to corroborate, after a fashion. There's a reasonable consensus that Corypheus was a tainted being that was once a Tevinter magister; there is a probable identification of him as Sethius, of House Amladris. Both of these things have appeared in
Inquisition and
WoT 2, loosely classified as likely but not certain. The fact that Corypheus was tainted but not exactly the same as other darkspawn emissaries, combined with his extensive knowledge of ancient Tevinter, makes it
likely that he participated in the attempt to reach the Golden/Black City.
But, assuming that that happened, there is not a single shred of positive proof, aside from Corypheus' own claims, that he ever actually got there. Nor is there proof corroborating his claim that the city was empty (or that there was a throne in the city and it was empty, depending on how you read that particular line of his).
It is impossible to falsify Corypheus' claims because there is no possible way to come up with any evidence to refute it. Similarly, there is no possible way to come up with any evidence to positively corroborate it.
This is simply the nature of the problem: no one else was there. I don't have any particular bias against Corypheus, nor do I have any particular leaning toward Andrastianism or any other Thedosian religious tradition you care to name, but you have to understand what you don't understand. We can't take Corypheus at face value about the orb or the Fade any more than we could take the Guardian of the Temple of Sacred Ashes at face value about Andraste.
In fact, the game itself drives home the problem of autopsy to the player by using the Inquisitor's own experience. Did the player encounter Andraste in the Fade, or not? What exactly was that spirit-Justinia anyway? Was it Justinia herself or the spirit who helped the player escape the first time? What about the second time? None of this is made particularly clear. Whatever the player decides is the correct answer, and how passionately the character believes in it, is up to the player. The game certainly doesn't provide a clear answer.