Nathaniel Schnier had notes — years’ worth of them. Having spent much of his childhood barely able to mutter more than a few words at a time, the young boy began speaking in full sentences at age 8. His first order of business? Setting the record straight. “He had a laundry list of everything that upset him throughout the years,” his mother, Dr. Kathleen Schnier, told The Post with a laugh. From disappointing dinners and denied desserts to long-held grudges against his older sister, it was