Formal.
As far as I know, there is no such thing as either standard or practiced diction. Informal is the way you communicate with your friends, for instance, which is not how you would write a thesis paper, legal document, etc.
2. They could add whatever they wanted, and make it to their liking. They could also make it healthier, and to fit their nutritional needs.
3. The health benefits of eating home cooked opposed to fast food is that it can be much healthier. In home cooked food, there is much less sugar, salt, and all the bad stuff. You wouldn't have to worry about preservatives.