Folding dry ingredients into the cake batter: at once or in 2/3 portions, does it make a difference?
You want the dry ingredients to be fully moistened and dispersed throughout the liquid with no dry clumps.  You also want minimal agitation so that you don't destroy the retrained air, introduced by the whipped egg whites, which acts to give a fluffier texture to the finished cake.

If you sprinkle the dry ingredients over the batter in multiple steps and gently fold in each addition, it requires very little agitation to moisten and incorporate; little more than lightly pressing over the surface with a spatula and a gentle fold or two.

If you add all at once, you have to mix the batter more vigorously to get the dry ingredients to fully incorporate.  This will cause the batter to deflate and will produce a denser cake.