Your numbers have a saturation index that is around -0.5 which is on the corrosive (plaster pitting) side, though not by that much, especially since this is with a pH of 7.2 and when the pH was higher it was more like -0.3 to -0.4. Perhaps over a long enough period of time this causes some pitting. You can use The Pool Calculator to calculate the Calcite Saturation Index. You can either target a higher pH of around 7.5 or can raise the CH and TA somewhat.
As for your other questions, I hope others can help you out.
Richard
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