They do have no mark but 1) we've had a good explanation for why that it works in TTT that doesn't involve the accuracy of their beliefs and 2) we've been told that their have been others who have used the Psukhe before the advent of Fane. So buying into the Fanim being right because their sorcery works has always been a palace built on air.
I presume you refer to the conversation of Kellhus and Achamian, where the first breaks down the metaphysics of sorcery and stuns Achamian?
I thought that bit, was just Kellhus making things up on the go. How would he know how Psukhe works, without having any knowledge of it and no experience? I understand that his awesome intellectual powers allowed him to see what Gnosis and Anagogic sorcery is, but Psukhe? I would think that is far-fetched even for Kellhus, but there you go.

As for the second point, I wasn't aware of that. It certainly isn't in the text, but I guess from a Q&A session.