> what qualifies as a measurement device
Anything, really. There are no "classical devices", after all, all of them are quantum systems, aren't they?
Anything, really. There are no "classical devices", after all, all of them are quantum systems, aren't they?
That idea is almost as old as quantum mechanics itself. (von Neumann, 1932 - if not earlier.)
There is also another weird asymmetry. For the spin we say that its superposition exists in my [slice of the] world but for the measurement device we say only half of the entangled state, the outcome I become aware of, is part of [my slice of the] world, what justifies this?
Nothing. It keeps existing, and (1/sqrt(2) * <I think spin is down|⊗<device shows spin is down|⊗<spin was down| + 1/sqrt(2) * <I think spin is up|⊗<device shows spin is up|⊗<spin was up|) keeps evolving, and since QM is linear, the summands evolve independently.
Yes, to the outside observer (the one who could look at the whole of the universe from the side/above without interacting with it) this does look very similar to MWI. But we are not outside of the universe, we exist inside it as a part of it, so for us it looks like spontaneous collapse of the wave function.
The "Copenhagen interpretation" as we understand it, in which some thing "collapses" when measured, trades under the same name but was invented later. As a result the original interpretations of Bohr/Heisenberg and their philosophy of science have somewhat exited the discussion, even though they had the only defensible epistemology -- the rest as you say is just ontology.