How Manson Everard Learnt About Two Causal Loops

In the case of Carl Farness, Everard:

had a hunch, an uneasy feeling that something was not right;
read Carl's reports;
steeped himself in knowledge of the Goth milieu;
roved that milieu from end to end, over and over.

(We want to read about Everard's endless roving.)

At last, he realizes that some poems, stories and traditions had their sources not in the mythical Wodan but in Carl's physical presence. These include the story of Odin appearing in a battle and betraying his descendants even though Carl has not done this yet...

In the case of the anti-Roman pagan prophetess, Veleda, Everard and Janne Floris follow Veleda back through time to the event that made her a prophetess - which was Janne rescuing Veleda from rape by the crew of a Roman ship.

Thus, whereas Everard had to deduce Carl's role, then persuade Carl that it was necessary to return to 372 and enact a betrayal, Everard himself was present, but unable to intervene, when Janne swooped down to kill the rapists - two very different ways to learn something.