I have never used baking powder in dumplings made with SR flour .. . they always come up OK ...but if you were using plain flour then half a teaspoonful would be sufficient .
I only ever use plain flour for anything, adding baking powder when needed. (It worked for my Grandmother)
4 & 2 is right for dumplings. The packet of baking powder tells you how mush to use for a particular dish.
No need to use baking powder just SR flour half the amount in suet and a good pinch of salt (important)
Two classic reasons for heavy dumplings are too much water: there must be just enough to form a light dough. and overhandling: Mix the water in and as soon as you get a firmish dough form rough balls and stick 'em in your stew!