Hi Ian,
I waited a while to read your essay as I thought, comprehensibility is a too incomprehensible expression in order to get something comprehensible out of it. I was wrong. It was a clear enjoyable read. A few questions however remained that I would like to ask and a few remarks I would like to make.
It seems to me, that you the context, under which a question can be comprehensive, must be classical. I completely agree, that the very meaning of a spin, implies the possibility to make an experiment along some direction and that we can know in which direction has been made (and share this information). This means that the directions must be somehow classically distinguishable. They can be constructed as coherent spin states. But could you imagine, that someone holds a superposition of two such 'classical' states as context and not being able to agree with you in which direction the spin measurement has been done? Or do you see limits in the applicability of the superposition principle?
I personally cannot really get some operation meaning from such a superposition, as normally the superposition is not an eigenstate of a generator of the group SU(2) representation except for the lowest non trivial representation 1/2. But then one of the main principles of the QM seem to have only limited applicability.
In a similar matter you seem to look for a reconciliation between the subjective view and an objective one. This seem to me is a problem brought to us mainly in QM and specifically by Wigner's friend type experiments. Does the stability of the universe also here play a role for the comprehensibility of the experiment?
The stability of the universe sounds for me extremely dramatic and I do not know what you exactly count as stable. I asked this last question, because in my essay, the condition for an experiment to be comprehensible (I call it definable or measurable in my essay), is to be unitary and symmetric. So if Wigner tries to measure his friend being in the superposition with the probe, he breaks unitarity and symmetry within the friends subsystem. So Wigner trying to make a incompatible measurement on his friend would be enough to destroy the stability of his friends lab experiment.
You actually do not answer the question of why the universe is comprehensible, but of under which conditions it is. I would be happy, if you would find the time to read and comment on my essay that has a similar topic. I ask under which physical condition physical concepts are definable.
Luca