Baking powder and salt.
Flax seeds
Certainly, white all purpose flour will work in muffins. Although they might turn out to be cupcakes, rather than muffins.
Baking soda helps muffins rise by reacting with acid in the batter to create carbon dioxide gas, which expands and makes the muffins light and fluffy. It also helps with browning and contributes to the overall texture of the muffins.
Muffins are fairly forgiving, if I was doing it, 1 teaspoon of oil, 2 tablespoons of water or milk and 1/8 teaspoon of baking soda or baking powder, whichever the recipe calls for.
Baking is a chemical process.
I have successfully substituted ground flax meal for wheat germ in some baking recipes with great success. I thought the flavour and texture was much better.
Yes. Elevated temperatures are harmfull for flax oils. That is also why flax is cold pressed.
You do if you want them to rise properly.
You can substitute baking powder for baking soda, but keep in mind that baking powder is not as strong as baking soda so you may need to use more. Additionally, you can try a mixture of cream of tartar and baking soda as a substitute.
It could be used depending on what you are baking. A better substitute would be Stevia.
Some of the oil can be replaced by applesauce, but not all of it.