It's probably worth noting that physicists are almost unanimous in their conviction that quantum weirdness needs no apologies. They use the theory every day, and it just works. As Feynman put it in one of his lectures, we have to accept Nature as she is, fundamentally absurd. Arguing against the veracity of quantum mechanics is a guaranteed way to be labeled a crank.
Maybe so. But as an outsider, I too have had trouble accepting that the universe can really work this way—that when you get down to the realm of the tiny particles that make up atoms, something can be here and there and everywhere, all at the same time. And being confronted with a quantum computer that exploits this synchronicity to do scads of simultaneous calculations finally makes the absurdity seem real. So, yes, I agree: This is as much a philosophical/metaphysical breakthrough as it is a scientific one.
I'm less sure whether the existence of one of these things resolves the confusion over just what quantum mechanics means. Some physicists call this "the word problem." The theory makes perfect sense mathematically, but those of us more comfortable with language (I guess we have a number problem) are doomed to argue over rival interpretations.
Consider the string of computing atoms that simultaneously tries out all the possible factors of the number 15. Each of these quantum calculations can be thought of as a little wave intertwined with other little waves, writhing and wiggling and finally settling into an answer. Or, if that metaphor grates, you can think of each calculation as a path through a maze. Unlike a laboratory rat or an ordinary computer, which must probe the pathways one at a time, the quantum computer can simultaneously traverse every twist and turn and immediately converge upon the prize.
Now, I agree with you that it is much harder to think of the calculations as having no existence other than as a wave of probability—a mathematical abstraction. How can an abstraction compute? But mathematically all these interpretations are said to be equivalent—just different attempts to translate precise equations into imperfect words.
But on to what you call the Shirley MacLaine question. Coincidentally (and I do believe it is only that), this morning's New Mexican, the hometown paper here in Santa Fe, reports that she is stepping down as official chairwoman of the New Mexico Film Advisory Board, which tries to lure moviemakers to the state. She will, however, remain "honorary chairwoman"—a quantum state in which she retains the honors but needn't attend meetings.
In the theory favored by David Deutsch, there is a parallel universe in which MacLaine retained the full responsibilities of her post, and another in which she was not appointed to the board in the first place, and another in which she never moved to New Mexico, and numerous universes in which she went into accounting or architecture instead of acting. (Some of us Santa Fe-ans wish we could move to the universe where her contractor had not plowed a dirt road halfway up Atalaya Mountain several years ago for a hilltop mansion she had planned to build.)
Far be it from me to say that someone who believes this "many universes" interpretation is crazy. Deutsch, an Oxford don, is certainly eccentric. Reputed to be nocturnal, he sleeps by day and works at night. And in the clear light of morning, he insists that a quantum computer must be doing each of its simultaneous calculations in a parallel universe.
Why doesn't he concede that his is just one possible interpretation? Well, suppose you had a quantum computer made from a string of 1,000 atoms, a speck too tiny to see. According to quantum theory, it could do more than 10300 calculations at the same time. That enormous quantity is far larger than the number of particles in this universe. So where, he asks, is all that calculating being done?