Good breakdown. #7 is not an explanation unless coupled with a hypothesis on why LW readers are more adept than mainstream philosophers and decision theorists at spotting the right answer on this problem. Unless one claims that LWers just have a generally higher IQ (implausible) an explanation for this would probably go back to #1 or something like it.

Personally, I think the answer is a combination of #1, #2, #3. I'm not sure about the relative roles played by each of them (which have a decreasing level of "rationality") but here is an analogy:

Suppose you know that there is a controversy between two views A and B in philosophy (or economics, or psychology, or another area which is not a hard science), that University X has in its department a leading proponent of theory A, and that bunch of theorists have clustered around her. It is surely not surprising that there are more A proponents among this group than among the general discipline. As possible explanations, the same factors apply in this general case: we could hypothesize that philosophers in X are exposed to unusually strong arguments for A, or that B-proponents disproportionately go to other universities, or that philosophers in X are slavishly following their leader. I contend that the question about LW is no different in essence from this general one, and that whatever view about the interplay of sociology, memetic theory and rationality you have as your explanation of "many A-ers at X" also should apply for "many 1-boxers at LW".