No, that is not how it works: I don't need to either accept or reject MWI. I can also treat it as a causal story lacking empirical content.

To say that MWI lacks empirical content is also to say that the negation of MWI lacks empirical content. So this doesn't tell us, for example, whether to assign higher probability to MWI or to the disjunction of all non-MWI interpretations.

Suppose your ancestors sent out a spaceship eons ago, and by your calculations it recently traveled so far away that no physical process could ever cause you and the spaceship to interact again. If you then want to say that 'the claim the spaceship still exists lacks empirical content,' then OK. But you will also have to say 'the claim the spaceship blipped out of existence when it traveled far enough away lacks empirical content'.

And there will still be some probability, given the evidence, that the spaceship did vs. didn't blip out of existence; and just saying 'it lacks empirical content!' will not tell you whether to design future spaceships so that their life support systems keep operating past the point of no return.

By that logic, if I invent any crazy hypothesis in addition to an empirically testable theory, then it inherits testability just on those grounds. You can do that with the word "testabiity" if you want, but that seems to be not how people use words.

There's no ambiguity if you clarify whether you're talking about the additional crazy hypothesis, vs. talking about the conjunction 'additional crazy hypothesis + empirically testable theory'. Presumably you're imagining a scenario where the conjunction taken as a whole is testable, though one of the conjuncts is not. So just say that.

Sean Carroll summarizes collapse-flavored QM as the conjunction of these five claims:

1. Quantum states are represented by wave functions, which are vectors in a mathematical space called Hilbert space.

2. Wave functions evolve in time according to the Schrödinger equation.

3. The act of measuring a quantum system returns a number, known as the eigenvalue of the quantity being measured.

4. The probability of getting any particular eigenvalue is equal to the square of the amplitude for that eigenvalue.

5. After the measurement is performed, the wave function “collapses” to a new state in which the wave function is localized precisely on the observed eigenvalue (as opposed to being in a superposition of many different possibilities).

Many-worlds-flavored QM, on the other hand, is the conjunction of 1 and 2, plus the negation of 5 -- i.e., it's an affirmation of wave functions and their dynamics (which effectively all physicists agree about), plus a rejection of the 'collapses' some theorists add to keep the world small and probabilistic. (If you'd like, you could supplement 'not 5' with 'not Bohmian mechanics'; but for present purposes we can mostly lump Bohm in with multiverse interpretations, because Eliezer's blog series is mostly about rejecting collapse rather than about affirming a particular non-collapse view.)

If we want 'QM' to be the neutral content shared by all these interpretations, then we can say that QM is simply the conjunction of 1 and 2. You are then free to say that we should assign 50% probability to claim 5, and maintain agnosticism between collapse and non-collapse views. But realize that, logically, either collapse or its negation does have to be true. You can frame denying collapse as 'positing invisible extra worlds', but you can equally frame denying collapse as 'skepticism about positing invisible extra causal laws'.

Since every possible way the universe could be adds something 'extra' on top of what we observe -- either an extra law (e.g., collapse) or extra ontology (because there are no collapses occurring to periodically annihilate the ontology entailed by the Schrodinger equation) -- it's somewhat missing the point to attack any given interpretation for the crime of positing something extra. The more relevant question is just whether simplicity considerations or indirect evidence helps us decide which 'something extra' (a physical law, or more 'stuff', or both) is the right one. If not, then we stick with a relatively flat prior.

Claims 1 and 2 are testable, which is why we were able to acquire evidence for QM in the first place. Claim 5 is testable for pretty much any particular 'collapse' interpretation you have in mind; which means the negation of claim 5 is also testable. So all parts of bare-bones MWI are testable (though it may be impractical to run many of the tests), as long as we're comparing MWI to collapse and not to Bohmian Mechanics.

(You can, of course, object that affirming 3-5 as fundamental laws has the advantage of getting us empirical adequacy. But 'MWI (and therefore also 'bare' QM) isn't empirically adequate' is a completely different objection from 'MWI asserts too many unobserved things', and in fact the two arguments are in tension: it's precisely because Eliezer isn't willing to commit himself to a mechanism for the Born probabilities in the absence of definitive evidence that he's sticking to 'bare' MWI and leaving almost entirely open how these relate to the Born rule. In the one case you'd be criticizing MWI theorists for refusing to stick their neck out and make some guesses about which untested physical laws and ontologies are the real ones; in the other case you'd be criticizing MWI theorists for making guesses about which untested physical laws and ontologies are the real ones.)

I am not super interested in having catholic theologians read about minimum descriptive complexity, and then weaving a yarn about their favorite hypotheses based on that.

Are you kidding? I would love it if theologians stopped hand-waving about how their God is 'ineffably simple no really we promise' and started trying to construct arguments that God (and, more importantly, the package deal 'God + universe') is information-theoretically simple, e.g., by trying to write a simple program that outputs Biblical morality plus the laws of physics. At best, that sort of precision would make it much clearer where the reasoning errors are; at worst, it would be entertainingly novel.