Not in all recipes; for most baking recipes substituting plain flour for cornflour will not work, since cornflour has no gluten (which is what makes dough springy) and it requires far more hydration. Adding some cornflour to the flour in baking will result in lighter baked goods, but only until the total flour is 5% cornflour; after that the baked good gets progressively more dense and inedible. Cornflour is useful for thickening custards, which plain flour is not so good at. But you cannot bake normally with cornflour. Both cornflour and wheat flour have a similar calorie content, wheatflour is lower in the glycemic load (GL) index compared to cornflour, and whereas cornflour is considered "highly inflammitory" (i.e likely to cause a reaction), wheatflour is only considered "inflammitory."
Not in all recipes; cornflour in custards cannot be replaced by normal flour (otherwise it will taste floury). In cakes some cornflour will add lightness, however there are plenty of cake recipes available that do not require cornflour. However in other situations you may be able to replace the cornflour with plain flour; alternatively you may be able to leave the cornflour out.
No. cornflour 'hardens' the mixture so it will keep its shape, normal flour doesn't do that.
The only alternative to cornflour I know is to mix equal parts of softened butter and plain flour to form a paste and use the paste instead of cornflour.
If I'm not mistaken cornflour is called corn starch in the States, in which case I would say: go for it. The difference will be that you will probably need less of it because it is a potent thickener.
You can use plain flour with a teaspoon of baking powder.
Yes you can, I use whatever flour I have to hand.
No, because the shortbread will rise. You can use plain flour or rice flour instead though. it would be really nice.
Yes, assuming that by "plain flour" you mean all-purpose flour. Because all three sound like the exact same thing.
Yes, it generally won't matter.
I'd use regular flour.
not really because they'd turn out like muffins
yes
Hot cross buns should be made using strong bread flour.Cornflour would not work as the main ingredient.The cross is usually plain flour made into a paste with water, cornflour could be substituted to make the cross.