Yes you can, I use whatever flour I have to hand.
You can use plain flour with a teaspoon of baking powder.
Yes, assuming that by "plain flour" you mean all-purpose flour. Because all three sound like the exact same thing.
No, because the shortbread will rise. You can use plain flour or rice flour instead though. it would be really nice.
No. cornflour 'hardens' the mixture so it will keep its shape, normal flour doesn't do that.
Yes, it generally won't matter.
That is 2.9 cups of flour
yes
The store mix has to list the ingredients and be specific. Wheat flour instead of Oat, Rice, Buck Wheat or some other type. Plain flour means all purpose, not self rising, cake or bread flour. All purpose or plain flour being wheat flour.
which is the best flour to make bread out of strong plain flour self rasing flour or plain flour
There is no difference between plain flour and all-purpose flour. They are one and the same. All-purpose (plain) flour does not contain the salt and baking soda that self-rising flour has.
AP Flour = All Purpose Flour = Plain Flour
not really because they'd turn out like muffins