I have not actually ever seen the Mermin device before, but am very familiar with the Stern-Gerlach apparatus that Mermin's device models. The Mermin device illustrates quantum superposition for single entangled particles just like the Stern-Gerlach device does for each particle and also illustrates the quantum measurement "problem" quite well too.
The challenge seems to be to explain in simple language quantum superposition and entanglement by using simple language to explain how the Mermin device works. The simple explanation is that the Mermin device Case B reveals the nature of quantum phase incoherence.
Our classical reality first of all mostly involves quantum phase coherence and all gravity relativity outcomes only involve quantum phase coherence. All of reality, however, actually does involve quantum phase and all matter bonds involve coherent quantum phases and coherent quantum phase makes up most of our classical reality as a result.
However, even bonded matter particles can exist with incoherent quantum spin phases above absolute zero, especially vaporized matter. For example, each silver atom in the vapor of the Stern-Gerlach experiment exists as an incoherent phase superposition of the silver atom's two quantum spin phases of +/-1/2.
Generally, classical reality can ignore quantum phase incoherence because of statistical averaging over large numbers of particles. However, certain quantum measurements of single atoms or particles do show the effects of quantum phase incoherence, which of course, has no classical explanation at all.
The Mermin device (see attachment) measures each of two silver atoms with entangled but still incoherent spin phases. When the two measurements have the same quantum phase or orientation, Case A, the two measured entangled spins always agree.
When the two measurements have different quantum phase at an orientation of 120 degrees from each other, Case B, the two entangled silver atom spin phases only agree 25% of the time. This means that each measurement of the two silver atoms with incoherent but entangled quantum spin phases has an incoherent precursor quantum spin phase that is only knowable with some well-defined incoherence.
Thus, quantum phase incoherence results in an intrinsic uncertainty for all quantum outcomes. Classical reality is a result of only quantum phase coherence and so it is actually quantum phase incoherence that reveals the true quantum nature of the world.
Correspondingly, the quantum measurement "problem" is only a problem because even many very smart people ignore the reality of quantum phase incoherence.Attachment #1: MerminExperiment.jpg