Is "πρόγευμα" a valid alternative for "πρωινό"? Which is more common?
Yes! πρωινό (or πρωϊνό) is much more common. You may see πρόγευμα only on a hotel/restaurant menu. πρόγευμα literally means before the meal. In Greece, lunch (μεσημεριανό - the midday meal) is considered the main meal of the day.