Thank you. "Line of Play" is the intended route that your ball is intended to go. You cannot place a mark along that route. Your feet are not along the line of play as you do not intend the ball to go through your legs. On the green, you all know that you cannot touch your line of play. If you take a legal putting stance, you are not standing on your line of play as everyone whoever putted would be taking a two- stroke penalty. You can lay a club at your feet but must remove it before you play your shot. There is nothing in the Rules about a dew mark as it in not on your line of play.
Yes, you can. You can brush away loose soil and sand as well.