Now, a royal insider is making an explosive new claim: Diana cheated first. In the upcoming documentary “King Charles: The Boy Who Walked Alone,” premiering on May 2 on Paramount+, several ex-girlfriends and former staffers spill new secrets about the royal family, ahead of the monarch’s May 6 coronation. Allan Peters, a former royal protection officer who worked with the family for nine years, appears in the documentary to share his perspective on Charles and Diana’s marriage, which lasted from 1981 until the famously unhappy pair separated in 1992 and divorced in 1996. “I can categorically say that the first...