While Pinellas-Pasco Medical Examiner Jon Thogmartin suggested yesterday that his autopsy of Terri Schiavo had cleared up most of the questions raised by critics of the court-ordered decision to starve her to death, renowned homicide detective Mark Fuhrman insists that key aspects of the case remain in doubt. Fuhrman, whose book on the Schiavo case, "Silent Witness," is due out on June 28, told ABC Radio host Sean Hannity yesterday that the most glaring mystery about the case remains unsolved: What was the cause of Schiavo's initial collapse on the night of Feb. 25, 1990? ..... (cont'd).... ..... "We [still]...