Washington — Secretary of State George Shultz flew home to brief President Reagan on the accord to get foreign troops out of Lebanon. If the agreement on withdrawing Israeli, Syrian, and Palestinian fighters from Lebanon holds, it would be the administration's first real foreign policy success. But it was under challenge on several fronts, especially from Syria, which said the plan would pose a grave threat to peace in the Middle East. The plan also came under pressure from the Soviet Union and Israel. In a move that has helped heighten concern over renewed crisis, 23 more wives and children of Soviet diplomats left Beirut, bringing to 110 the number of Soviet dependents who have quit Lebanon this week. In Jerusalem, meanwhile, the Israeli government came under attack in the Knesset (parliament) for failing to win better security guarantees in the agreement. Opposition leader Shimon Peres said it was doubtful the plan had improved the country's position.