1), sorry not sure on this one. I have used fruit essences, which are steam-distilled instead of extracted in alcohol, but I’m not sure about vanilla essence.
2) Always use unsalted butter. There are two big reasons, first, salt preserves the butter so the unsalted tends to be fresher and frequently better-tasting. Second, there is no standard as to the amount of salt in the salted variety, so it is difficult to know how much salt you are adding to a recipe, it could be way too much or not nearly enough.
3) Most cake recipes need adjustment to increase structure if they will be baked in a larger pan, it is not always successful to just double the recipe. Often this is accomplished by decreasing the baking powder (see section on Rose Factor in “The Cake Bible”). Even if doubling did work, it would not be correct to double the baking time. A 9” layer cake recipe increased for 10” layers might only need an additional 10 minutes. A heat core and Cake Strips might also be needed, depending on the shape of the pan, to keep the edges from overbaking before the center is done.