LONDON — What was scheduled to be a straightforward 10.5-hour transatlantic flight turned into a nearly 20-hour ordeal for travelers aboard a Delta Air Lines flight from Paris to Seattle, after a mechanical malfunction involving an onboard lavatory forced the aircraft to divert twice before reaching its final destination. Delta Air Lines Flight DL81, operated by an Airbus A330-900neo (registration N410DZ), departed Paris Charles de Gaulle (CDG) …