You could still have worn synchros, but that won't prevent shifting out of gear with the engine off. The synchros simply normalize adjacent gear speeds during shifts to prevent grinding. I forgot to mention that if you are parked on a hill or just on a slight incline and in gear to prevent the vehicle from moving, then you will likely not be able to shift out of reverse with a clutch that fails to release. This is because the reverse gear has pressure on it. To relieve the pressure, you may simply jack up one of your drive wheels. Make sure to anchor your vehicle to prevent it from rolling with a jack under the car.
Good luck.