The leaders of the State Department’s Benghazi probe defended their inquiry into the 2012 attack, but they acknowledged to Congress on Thursday that their mission was limited in scope and faced questions over why they gave Secretary of State Hillary Rodham Clinton an advance look at their findings. Retired Adm. Mike Mullen, vice chairman of the accountability review board, also acknowledged that he had warned Mrs. Clinton’s chief of staff not to send a particular official to Congress because he thought “she would be a weak witness” who might have hurt the State Department’s stance. Republicans said those moves called...