Right in the beginning of the sequence you managed to get phases wrong.

Hopefully this mistake will be fixed one day, so the sequence will be judged on the merits of the argument it presents, and not by the presence of a wrong factor of "i".

given relativistic issues in QM as described, QM is just approximation which does not work at the relevant scale, and so concluding existence of multiple worlds from it is very silly.

Nonrelativistic QM is an approximation to relativistic QFT, and while relativity certainly introduces a new problem for MWI, it remains true that QFT employs the superposition principle just as much as QM. It's a formalism of "many histories" rather than "many worlds", but the phenomenon of superposition, and therefore the possibility of parallel coexisting realities, is still there.

I would agree that it was foolish for Eliezer to flaunt his dogmatism about MWI as if that was evidence of superior rationality. What I would say is that he wasn't worse than physicists in general. Professional physicists who know far more about the subject than Eliezer still manage to say equally foolish things about the implications of quantum mechanics.

What the evidence suggests to me is that to discover the explanation of QM, you need deep technical knowledge, not just of QM but also QFT, and probably of quantum gravity, at least to the level of the holographic principle, and you also need a very powerful imagination. Possibly the correct answer is a variation on a concept we already possess: many worlds, Bohmian mechanics, loops in time, a 't Hooft cellular automaton. If so, then the big imaginative leap was already carried out, but the technicalities are still hard enough that we don't even know that it's the right type of answer. Eliezer-style dogmatism would be wrong for all the available explanations: we do not know which if any is right; at this stage there is no better strategy than pluralistic investigation, including hybridization of these supposedly distinct concepts. But it's also possible that the correct answer hasn't yet been conceived, even in outline, which is why imagination remains important, as well as technical knowledge.

If you accept this analysis, then it's easier to understand why interpretations of quantum mechanics present such a chaotic scene. The radical ontological differences between the candidate explanations create a lot of conceptual tension, and the essential role of subtle technicalities, and mathematical facts not yet known, in pointing the way to the right answer, mean that this conceptual tension can't be resolved by a simple adjudication like "non-collapse is simpler than collapse". The possibility that the answer is something we haven't even imagined yet, makes life even more difficult for people who can't bear to settle for Copenhagen positivism - should they just insist "there must be an answer, even if we don't know anything about how it works"?

It's therefore difficult to avoid both dogmatic rationalization and passive agnosticism. It's the sort of problem in which the difficulties are such that a return to basics - a review of "what I actually know, rather than what I habitually assume or say" - can take you all the way back to the phenomenological level - "under these circumstances, this is observed to occur".

For people who don't want to devote their lives to solving the problem, but who at least want to have a "rational" perspective on it, what I recommend is that you understand the phenomenological Copenhagen interpretation - not the one which says wavefunctions are real and they collapse when observed, just the one which says that wavefunctions are like probability distributions and describe the statistics of observable quantities - and that you also develop some idea of what's involved in all the major known candidate ontologies.

For readers of this site who believe that questions like this should be resolved by a quantified Occam's razor like Solomonoff induction: in principle, your first challenge is just to make the different theories commensurable - to find a common language precise enough that you can compare their complexity. In practice, that is a difficult enough task (on account of all these ideas being a little bit underspecified) that it couldn't be done without a level of technical engagement which meant you had joined the ranks of "people trying to solve the problem, rather than just pontificating about it".

The muckiness surrounding the interferometer is well-known; in fact, the PSE question was written by a LWer.

Rest of the argument... given relativistic issues in QM as described, QM is just approximation which does not work at the relevant scale, and so concluding existence of multiple worlds from it is very silly.

The conclusion isn't "MWI is true." The conclusion is "MWI is a simpler explanation than collapse (or straw-Copenhagen, as we in the Contrarian Conspiracy like to call it) for quantum phenomena, and therefore a priori more likely to be true."

And yes, it is also well-known that this quote is not Yudkowsky at his most charming. Try not to conflate him with either rationalism or the community (which are also distinct things!).