I think your (b) is what's most relevant here.
That is, I generally interpret the recurring "what if your thought leader is wrong?" threads that pop up here as an expression of the expectation that reasoning cannot be judged (either in the specific context, or more generally) except through evaluation of the results it endorses after they become actual.
There are varying interpretations I make of that expectation, depending on specifics and on how charitable I'm feeling.
Some of those interpretations I even agree with... for example, I would agree that it's much easier for me to fool myself into inaccurately thinking a line of reasoning either makes sense or doesn't make sense, than it is for me to fool myself into inaccurately thinking that a specific prediction either happened or didn't happen. (Both are possible, and I've likely done both in my time, but the latter is more difficult.)
I blew through all of MoR in about 48 hours, and in an attempt to learn more about the science and philosophy that Harry espouses, I've been reading the sequences and Eliezer's posts on Less Wrong. Eliezer has written extensively about AI, rationality, quantum physics, singularity research, etc. I have a question: how correct has he been? Has his interpretation of quantum physics predicted any subsequently-observed phenomena? Has his understanding of cognitive science and technology allowed him to successfully anticipate the progress of AI research, or has he made any significant advances himself? Is he on the record predicting anything, either right or wrong?
Why is this important: when I read something written by Paul Krugman, I know that he has a Nobel Prize in economics, and I know that he has the best track record of any top pundit in the US in terms of making accurate predictions. Meanwhile, I know that Thomas Friedman is an idiot. Based on this track record, I believe things written by Krugman much more than I believe things written by Friedman. But if I hadn't read Friedman's writing from 2002-2006, then I wouldn't know how terribly wrong he has been, and I would be too credulous about his claims.
Similarly, reading Mike Darwin's predictions about the future of medicine was very enlightening. He was wrong about nearly everything. So now I know to distrust claims that he makes about the pace or extent of subsequent medical research.
Has Eliezer offered anything falsifiable, or put his reputation on the line in any way? "If X and Y don't happen by Z, then I have vastly overestimated the pace of AI research, or I don't understand quantum physics as well as I think I do," etc etc.