Is annealing nickel plated brass ( by your candle method) any different with nickel, or any more or less effective?

So far it's worked the same way. Nickel plating is normally very thin compared to the brass itself.

Must also point out, again, that the candle method was developed by Fred Barker. I just reported on it--after he sent me a copy of an article he published in the late, lamented PRECISION SHOOTING magazine.

OK thanks compadre!

Is there a purpose for nickel plating cases? Or is it just for appearance?

Corrosion resistance, especially if cartridges are to be carried in leather loops

And you can find your brass easier in the field! That can help sometimes.