Thanks for the vote of confidence. I would not worry too much about magnesium. Since the ABC+ contains Mg, the likelihood of it being low or unbalanced is rare. I test for Mg once or twice per year. If you dosed Randy's 2 part or kalkwasser as your only means for supplementing Ca/alk, then you would need to test more often.
As far as your concerns, usually alk does not creep up and Ca remains the same. Bicarbonate is used up quicker in the short term by corals relative to calcium. It is my opinion that the ABC+ is not exactly balanced favoring calcium. Alan at Red Sea assured me that it is balanced, however I've noticed that alk always needs an extra dose at the end of the week. So my guess is that your concern is just from an error in testing. Either way, it's not a big deal. The directions on the label say to dose for a week and correct any imbalances with their one parts. I just use baking soda for the alk part but there is a Red Sea buffer in powder or liquid form. Same for Ca and Mg.
Do you have unwanted algae in your tank? If so, I would keep the Nopox dosing the same. If not, I would only dose 1ml of Nopox daily and test again in a weeks time. I would also dose 10ml of Reef Energy daily. If nutrients are that low, the corals need the RE.
You (almost) never want to stop the Nopox completely as you can crash your bacterial population. Weening off is the best way.