Dear Jochen,
such a great essay. Fascinating to reconstruct quantum mechanics from "epistemic horizons".
There are a few points that escaped my understanding in your essay and I would like to use the chance of this blog to ask a few questions and make some remarks.
Classical physics worked pretty well for a few hundred years (and still does) for many phenomena. Also measurements can be described with classical physics. Quantum mechanics came in slowly in the attempts to explain the blackbody radiation and the discrete atomic spectra and other phenomena. None of these connected directly to limits of measurement or knowability. The point I want to make is: If classical physics/science is principally possible, where did the 'quantum' sneak in, in your argument? Such that the quantum would become necessary for epistemic reasons. I have not seen your two principles of section 1 in your prove by contradiction in section 2.
I sympathise with the aim to use an epistemic horizon for some arguments about the structure of laws or even reality (whatever this means). Specially because I belief that the vieew that things, properties and laws that are completely independent of the relations of the things with the rest is overly onesided. However you certainly know the quote from Einstein, when Heisenberg went to him and told him, that Einstein's theory taught them that only observable elements should enter the theory. Einstein replied that it was the other way around. It is the theory that tells us what can be observed. This means for me, that to use an epistemic horizon of what can be know, must at least be justified.
To advertise my essay: I came to a similar conclusion as you regarding the EPR experiment. You wrote: "Only given that one has actually measured xA is reasoning about the value of xB possible." In my essay I wrote on page 6: "But the very same experimental setup (EPR), shows that the setting of the reference frame far away from the object determines the possible, defined propositions."
Luca