Falcon 9 has already actually proven engine out capability during a flight where a leak shut down an engine. Primary mission was completely successful and secondary would have if Nasa had relaxed rules about ride-sharing ISS flights.
Return should actually be easier. Ascent from Mars takes less delta-v due to less gravity and drag. Return to Earth takes less than going to Mars because you aren't fighting the sun's gravity. Re-entry and landing at Earth requires less because you have a much more dense atmosphere that you can use to brake.