You comment: "You really need the original multiverse
to explain how a quantum computer can perform a
quantity of parallel computations that exceed the
number of particles that comprise it (or even exist
in our universe)".
I would say: When a "quantum computer" actually
exists, then we might be able to study it empirically
and rationally decide upon the need or lack thereof
for a "multiverse" or a "many worlds" theory in order
to explain how it functions.
Until the mythical "quantum computer" actually exists,
it is just one more pseudoscientific myth that is hobbling
science and eroding science's credibility.