First, regarding the idea of magical universes or quantum immortality for that matter, doesn't this assume a truly infinite number of universes? However, if you start with the idea that the reality we experience is being created by a mechanical/computational process, isn't it more likely that the number of universes is just extremely large? Why should we assume the "creator" (however you choose to define that) has access to infinite resources? Also, everything that makes up our universe appears to have finite characteristics (per QM), so it seems like every possibility within the parameters of the multiverse could be covered by an enormous, but not infinite range of possibility.
My understanding of QM is that it describes possibilities (even if vanishingly small) of bizarre things occurring in our everyday world. For instance, I once read a book in which the author calculated the possibility(incredibly small obviously) that our planet would suddenly appear in orbit, fully intact, around another star. He argued that QM allows for this possibility.
I think we are overlooking something here. It seems like there should be a quanta of probabilty, just as there is (apparently) with time, space, and matter. In other words, once the probability of something happening falls below a certain threshold, it is not realized. Could there be a Planck scale of probability? Does decoherence somehow keep these strange events from occurring on a macro scale?
Also, it seems to me that the violation of other physical laws comes
into play in preventing many scenarios from taking place. For
instance, with quantum immortality, I understand the concept that if
there are infinite copies of me, there will always be one more
universe in which I survive another second. But the reality is that
there would seem to be a rate of diminishing return here. The
probability curve would have a point where it approaches zero, even as
the number of alternatives approached infinity.
Another way to resolve the immortality issue is to presume
consciousness survives death, but I will not remark on that further.
One thing that I think hurts the MWI as a theory is the misconception among many that everytime a choice is made, the entire universe splits in two, and there is a proliferation of all of these virtually identical copies of universes out there somewhere. In reality there is only one universe, and there is a proliferation of differences being created. The only thing that matters are the recorded differences, everything else remains unchanged. If you view our reality as a virtual reality it is much easier to understand this concept. For instance a program that predicts the weather doesn't have to create an entirely new simulation for each outcome it predicts- it can overlap the various possibilities in one simulation.