I'm aware of the theory of the "big lie", and the fact that neurons that fire together, wire together...
But what about Feynman's injunction: "The first principle is that you must not fool yourself -- and you are the easiest person to fool." Is that kind of empirical principle just out of reach for the standard human?
In terms of repetition, I think David Hume's theory of causal inference is pretty relevant too.
Just because something is repeated regularly, e.g. seems to happen every day doesn't mean that on the day when it doesn't happen, our heads explode because we have built a rigid expectation. Instead we can usually discern possible, exceptional, reasons why it isn't happening.
I'm totally in agreement with you on the role of repetition in eroding the landscape of memory to create convictions. We might rarely remember a one off event that is never referred to. But you seem to be claiming that there's no higher-level activity, or even possibility of higher-level activity involved in mentally evaluating what to believe.
The famous Star Trek scene where Picard is encouraged/"brainwashed" to lie about the number of lights he sees comes to mind. He resists and continues to report what his senses tell him. This cultural theme of reason resisting the repetition of lies is a big one, and you haven't really presented any convincing evidence to discredit it.