Yes, assuming that by "plain flour" you mean all-purpose flour. Because all three sound like the exact same thing.
You can use plain flour with a teaspoon of baking powder.
Yes you can, I use whatever flour I have to hand.
YEs you can
No, because the shortbread will rise. You can use plain flour or rice flour instead though. it would be really nice.
white and yellow
Yes, it generally won't matter.
Use wholemeal flour instead of white - OR - replace some of the white flour with bran.
No. cornflour 'hardens' the mixture so it will keep its shape, normal flour doesn't do that.
"White flour" is wheat flour, it's just been processed in a slightly different way. It's perfectly fine to use whole wheat flour instead of white flour in a pizza crust.
not really because they'd turn out like muffins
With most flours if it doesn't specifically say "cake flour" on the package, then it is not the same as cake flour. But you can substitute plain (not self rising) flour for cake flour. For each cup of cake flour called for in the recipe, use one cup minus two tablespoons plain flour, and sift it at least twice (after measuring). White Lily says you can substitute their flour for "cake flour" without making any changes, and it does not need to be sifted. For more information on White Lily Flour see the related link below.
Yes, you can use plain flour instead of cornflour in lemon meringue filling, but the texture may differ slightly. Cornflour helps achieve a smoother, more delicate filling, while plain flour may result in a thicker and slightly less silky consistency. If using plain flour, be sure to cook it thoroughly to eliminate any raw taste. Adjust the quantity as needed, as plain flour may require slightly more than cornflour.