Downtown Disney is open until 2 so it's the latest "safety net". Because of the Downtown Disney, the internal campground busses run until 2:30 AM so you should be good.
Here's a little known "back door" if it's raining and you have to come back from Magic Kingdom. You can skip the boat. You can take the bus to the Wilderness Lodge. Once at the Wilderness Lodge, you can take the bus to Fort Wilderness. The bus from the Wilderness Lodge to campground is actually the Campground Orange Loop internal bus. It goes from the settlement over to the lodge and back before continuing up the middle of the campground. You can catch Orange bus, ride it to the Wilderness Lodge and catch the bus to MK without walking more than 20 steps. The boat is great but if it's scortching hot or raining, this little trip may be more comfortable.