Hi Ian.
I enjoyed this.
Your conclusion is remarkably similar to mine, though you get there through a different route.
Couple of questions/comments:
You write that "Other aspects might be unknowable because the universe's fundamental fabric is such that no machine can be constructed to produce a correct truth value for some truth-conditional statements."
This seems to imply some realm of "things in themselves" (independent of context/questions/etc). I doubt Wheeler would have thought that, being influenced by Bohr and all. "It from Bit" also means "No ifs if no Bits".
You also write: ""Yet it is wrong to say that there was any change in the underlying physics between then and now. What changed was our knowledge of that physics, i.e. we increased our information."
Well now: given what you say, the fact that contexts have changed, so that the kinds of questions we put have changed (on which we must agree to generate objectivity), you might say that the physics has changed too as a result of that. In Eddingtonian terms, the "Physical Universe" changes between then and now.
Best
Dean