Oats/Bitul

Question

Sholom Aleichem Rabbis Abadi, 1. I thought the five grains that can become chometz are wheat, barley, oats, spelt and rye. In permitting oatmeal for Sephardim, are you saying oats are not part of the five? What's the fifth grain, then? 2. According to Ashkenaz custom, doesn't chometz that was batul beshishim before Pesach "return and awaken" when Pesach starts so that even a mashehu makes the whole thing chametz? 3. Rabbi CYA wrote that corn syrup is batul in a drink because kitniyot are batul b'rov. Is this so even if the corn syrup gives (sweet) flavor to the drink? 4. As has been pointed out by readers on this site, corn (like peanuts, etc.) is a New World produce. As such, how did corn fall under the category of kitniyot, since it was not around in Eastern Europe until at least the 1600s? Chag Sameach. Thank you for your time.