By David Shepardson WASHINGTON (Reuters) -An investigation by the Federal Aviation Administration (FAA) found that the agency failed to conduct proper safety oversight of Southwest Airlines, a federal watchdog said Wednesday. The Office of Special Counsel (OSC) made public an April FAA report that came after four whistleblowers alleged the U.S. aviation regulator “with influence from Southwest, fast-tracked reports on dangerous incidents involving pilot error” at three U.S. airports. The OSC said FAA’s investigation confirmed it had mishandled those investigations and “carried out recommended …