mixing tap and distilled is a pretty decent solution to that problem if you can recognize the symptoms of ca def vs other nutrient lockout.
Keep and eye on the pH, since it tends to rise over time with RW and hydroton- it seems to take 4-7 days to stabilize after the system is first 'wetted' and that can contribute HUGELY to a high pH problem.
You're right about the Na+.
And you are also correct in your labels of the oxidation states of the metals... haha cool... nerds = woot. (I'm a lab rat by profession, analytical chem specifically)