I haven't looked at the discord in ages. I assume whomever you are speaking about probably watched Jude's video because he goes into Gunther's pretty varied catalog of books. Gunther seemed capable of writing about any topic he set his mind to, which is why I'm very suspicious that he was pulling one over on all of us.
Vordahl as Clara! HA! Now that's funny. Barb is indeed being investigated as a possible hoaxer and there are some plausible connections, but that's not for me to divulge. I haven't looked too much into it anyways.
My current stance is that it smells like performance art. I think saying he "lied" is an unnecessary pejorative, at least as far as my view of him. It'd be like saying Andy Kauffman was "lying" or that the Coen Brothers were "lying" about Fargo. Artistic license is the way I'd phrase it. That seemed to be the majority opinion in the reviews I've read from the time period. It was sort of a "wink-wink" thing i.e. "we see what you're up to and it's quite clever".
As for my legal cases, you can usually tell when someone is full of shit pretty quickly. Just an intuition I guess. But you said, there are some damn good liars out there. Most of my clients don't lie to me because they know I have the evidence. I have had a few who I totally believed UNTIL I got the surveillance footage showing them doing what they said they weren't doing! Lying is a skill I suppose. Also, motivation matters as well. If the person has no motivation to lie and gets a fact or two wrong, well, maybe that can be forgiven. Like say if it's just a random witness to something. Take Robert Gregory in the Cooper case. Is his testimony useless because he says Cooper sat in the window seat and says that a dark headed stew sat with him the majority of the time? No, I think not.
On the other hand, we have an evidentiary rule that's called The Fruit of the Poisonous Tree. This usually applies to law enforcement and their investigation. If they screw up one thing, then by default you have to assume that the rest of it is screwed up or susceptible to being screwed up. So it's really a case by case basis of when someone's testimony should be deemed useless.
With the Gunther book it's difficult because we don't know anything about Cooper before or after the hijacking, so everything that "Clara" says outside of the actual hijacking is unfalsifiable. Yet she fails on the very few things in the book that we CAN check. She said he had a reservation as opposed to just buying his ticket, she has him seated in row 15, Capt Scott coming back to check the bomb, he asks the pilots their location before jumping, etc. And yes, I know that she wasn't there for the hijacking, but she wasn't there for his life before she met him either, and she appears to have supplied a ton of details about that stuff. So why should that be trusted? I don't know, she just seems like a literary device to me, as Georger said. It's like the way Melville uses Ishmael in Moby Dick or something.
But again, just my two cents.