Thank you for your question. Thank you especially for sharing your reasoning, which I believe is on the money.
I agree that you would follow rules for abbreviations in transcribing the single letter chemical symbols you mention (N for Nitrogen, N[superscript 3 minus] for the anion of Nitrogen that has three extra electrons, O for Oxygen, etc.) By this logic, yes, you would also need an English letter indicator for the O in "3 mol O = 3×16.00 g = 48.00 g O".
If I find a really good reference for this (or really good reference against this), I will post here again.