You can not leave the starting section of Broken Valley without defeating the ghost under the church in the village. After the ghost is defeated you can leave the church and return later.
For the destroyed chapel with the entrance to the secret tunnel you can only enter that way until the quest is complete, but as Hassat Hunter said, you can get back in from the hatch near the farm. In D2:ED there could be a cave-in in the passage at a certain point, preventing you from getting back. However, if you mindread the prisoner in the cell before heading to the passage you would arrive to find bandits already breaking him out of the cell (getting the option to let them leave or fight), and there would not be a cave-in. In DKS I avoided mindreading the prisoner, but there was no cave-in.
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