"I still need to defend the claim that (a) the questions we’re discussing, centered around quantum mechanics, Many Worlds, and decoherence, and (b) the question of which physical systems should be considered “conscious,” have anything to do with each other."

There is no reason to assume that they do. The whole question is probably meaningless. There's no reason to treat the (as yet almost completely understood) computations called "consciousness" any differently from any other computations.

All the existential questions about parallel execution of conscious computations exist in the absence of quantum mechanics. What happens if you calculate the state of your brain N times, with very slightly different initial states, and choose the best (happiest, most correct, whatever) resulting state. What happens to the versions of you that didn't get selected? Are they the same person? Did you die N times? Does it matter?

What if you run the calculation is a perfectly classical sense N times, with the same initial state, so the end state of the computation is the same. Did the same identical consciousness die N times?

The morality is difficult. It could be horrible. That's not a problem with the theory, the objective morality of the real world is horrible. The universe we're in is built on uncountable terrors, we wouldn't exist without uncountable trillions of conscious beings dying horribly, but arguing that terror and torture is only real if there is evidence of its occurrence is worse. It seems cowardly to me.

I've already addressed the statistical argument for Boltzman Brains in another thread, and I feel that the whole argument rests on a deep confusion between local and global entropy. You'd do better to worry about Roko's Basilisk.

Finally, in reply to "From this extreme, even most scientific rationalists recoil.  They say, no, even if we don’t yet know exactly what’s meant by “physical instantiation,” we agree that you only get consciousness if the computer program is physically instantiated somehow." I give you Greg Egan's novel "Permutation City".
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