Thanks tkamiya for your excellent elaboration, I understand what you mean now. Odd sodium sulfite behaviour indeed. Perhaps the weaker-the-faster rule is, in its opposite incarnation, indeed responsible for the undoing of the sulfide toning that I observed with MCC 110. But I've decided to chuck the sodium sulfite altogether and just go with water instead.
Also, after seeing what sulfide does to Ilford MGWT I don't think I will bother using sulfide with MCC anymore. Ilford MGWT seems made for sulfide toning. MCC pales in comparison (not literally though).
I like your drop-in-the-ocean reasoning, quite elegant and very understandable. Sounds like you and Roger will have some more pondering and debating ahead of you .