I love oyster mushrooms, and they are so easy to grow. As they did in your experiment. The fact that slugs ate them is beside the point. Good for them, but too bad you lost out on it. Though once inoculated, the mycelium should weave through the entire trunk, so even if it gets flooded it should survive, and come to fruit when the time is right. Good luck for the next time! I bet you will have a mushroom feast before soon!