The methodological error is this: social psychological theories are descriptive, while Bayesian probability is prescriptive, it's an epistemic optimum.
You don't get to say "he reduction of implausibility to cognitive dissonance bears significant philosophical weight. It denies both Bayesian and coherentist theories of knowledge", because one is about how things are, the other about how things should be.
Sometimes the search for dissonance reduction leads to truth, and sometimes it leads away from truth. Rationality is a limiting case of dissonance reduction, but it's one impossible to specify except from within a psyche subject to the laws of cognitive dissonance. Then this problem: how do we even express the expectation that scientific theories get closer to the truth and religious theories do not? We can say that scientific theories depend on experimental and observational practices and therefore have at least the possibility of resting on actual evidence. We can say scientific theories have greater plausibility than religious theories, these both being judgments that are a product of the law of dissonance. But, counter-intuitively (at least for me), we can't say that scientific theories are more probable than religious theories. It isn't, it's important to notice, that we don't know which is more probable. Rather, the whole notion of probability as applied to theories is misbegotten. That a theory is implausible or plausible is a far-mode conclusion, and far-mode doesn't deal in the relative frequencies modeled by the probability calculus.