Could I use flour instead of cornstarch?

Totally. Let it sit for a couple of hours if you'd like. That will give the cornstarch more time to absorb the oil or grease.

Yes, you can use all-purpose flour. However, the cake will not rise and won't be as fluffy as it would be with self-rising flour.

In this recipe, the bread flour has a higher gluten content, making the bread have a more chewy and elastic texture.

Yes, and they cookies will turn out fine; rice flour is just more ideal as it makes a better cookie.

You can use coconut or almond flour, though you might have to adjust the other ingredients in the recipe.

You can use a tarp or any plastic sheeting, just be sure the material is strong and won't tear.

Technically, the King Arthur Sir Lancelot flour (their signature high-gluten flour) and standard bread flour are different flours due to protein content. I've used just the Sir Lancelot flour as...

Yes, you can do that as well.

Yes. This fits the 25 percent formula for baked goods. A higher substitution percentage is allowed for things like crusts.