You're arguing about something that seems interesting and possibly important, but it doesn't sound like the mathematical likelihood of the theory. Eliezer starts from a Bayesian interpretation of this number as a rational degree of belief, theoretically determined by the evidence we have. As I understand it, this quantity has a correct value, and the question of how much the theory explains has a definite answer, whether or not we can calculate it. The alternate Discordian or solipsistic view has much to recommend it but runs into problems if we take it as a general principle.

Now run time has no obvious effect on likelihood of truth. I don't know if message length does either, but at least we have an argument for this (see Solomonoff induction). And the claim that MWI adds an extra postulate of its own seems false. MWI tries to follow Occam's Razor -- in a form that seems to agree with Solomonoff and Isaac Newton -- by saying that no causes exist but arrows attached to large sets of numbers, and the function that attaches them. Everything you call magical or imaginary follows directly from this.

Before moving on to the problem with this interpretation, please note that Bayesianism also gives a different account of "unobserved things". Some of them, like aether and possibly absolute space, decrease the prior likelihood of a theory by adding extra assumptions to the math. (Eliezer argues this applies to objective collapse.) Others, like Santa Claus, would increase the probability of evidence we do not observe. This has no relevance for alternate worlds. The evidence you seem to want has roughly zero probability in the theory you criticize, so its absence doesn't tell us anything. The argument for adopting the theory lies elsewhere, in the success of quantum math.

Now obviously the Born rule creates a problem for this argument. The theory has a great big mathematical hole in it. But from this Bayesian perspective, and going by the information I have so far, we have no reason to think that whatever fills the hole will reduce the number of "worlds" to exactly one, any more than we have reason to believe in exactly 666 worlds. It really does seem that simple. And from what I've managed to read of Feynman and Hibbs the authors definitely believe in more than one world. ("From what does the uncertainty arise? Almost without doubt it arises from the need to amplify the effects of single atomic events to such a level that they may be readily observed by large systems." p.22) So I don't think my simple view results from ignorance of QM as it existed then.