Yes margarine and vegetable shortening can be used to substitute butter. Butter is mostly used to add density and help whatever you're cooking rise evenly.
No, peanut butter and butter are very different things. They don't substitute for each other. While peanut butter is thick and sweet , and butter is not thick , and salty , they are almost opposites .
Yes and no. When I make chocolate chip cookies, I usually replace half of the butter required with peanut butter. It gives it a peanuty taste and hopefully makes it a bit healthier cause the fats are...