1. Can a coin found in pocket change ever be graded as mint state? Yes, it's possible, but any such finds would typically fall in the lower MS range. Imagine receiving change at a store and getting home to find a 'pristine' MS64 2018 cent or 2006 quarter. It does happen, but not every day.
3. I know that you're never supposed to clean a coin because it loses value but buffing a coin with a soft cloth leaves it lustrous and bright and isn't that the quality that often brings value to a coin? Not necessarily, shiny does not always equate to value--many, many other factors come into play, such as very low mintage, relative rarity, errors and varieties, etc. Buffing coins is not a recommended practice. Suggest searching this site for conversations about coin cleaning.
3. I know that you're never supposed to clean a coin because it loses value but buffing a coin with a soft cloth leaves it lustrous and bright and isn't that the quality that often brings value to a coin? Not necessarily, shiny does not always equate to value--many, many other factors come into play, such as very low mintage, relative rarity, errors and varieties, etc. Buffing coins is not a recommended practice. Suggest searching this site for conversations about coin cleaning.