U.S. Marine Lt. Col. William R. Higgins, abducted a year ago Friday while serving as a U.N. observer, is alive despite his captors' statement that he was sentenced to death as a spy, a Shiite Moslem leader said in Tyre, Lebanon. "He'll be released eventually" by the pro-Iranian Shiite extremists who seized him, Abdul Majid Saleh, political officer of the Amal militia in this ancient port 50 miles south of Beirut, said Thursday. Higgins, from Danville, Ky., commanded a 76-man U.N. observer group monitoring the Lebanon-Israel border. He was kidnapped bygunmen who stopped his car, 20 minutes after he had met with Saleh, on the highway south of Tyre. Saleh would not say how he knew Higgins had not been killed bythe Organization of the Oppressed on Earth, which claimed responsibility for theabduction.