Unfortunately you can not get back into the secret passage after the cave-in (though if you had mindread the prisoner in the cell previously, but not told Louis anything when accepting the quest, then that encounter would have been a little different, and there would not have been a cave-in).
According to the
walkthrough, that key doesn't appear to have a corresponding lock. I picked up and used keys, but never tried to keep track of which went with which (though many are obvious).
Did you go back to talk to Folo in the village? I went to the farm first, and tried a couple different solutions to Carl's quest, but he never had anything to say about the pigs. I didn't release the pigs after getting him arrested, though (went with the combat option to get the necklace), so am not sure if there could be a bug with that. He is not needed for anything else, so it shouldn't matter if he 'escaped'.
Welcome to the forum.
