The mystery in question is The Good Men of Sherwood. I have seen online the way to complete this mystery but I cannot execute this in game as the characters are not doing what they are meant to be doing, and one of them seems to be dead, through no fault of my own. I cannot speak to any of the NPCs or otherwise get them to acknowledge me to begin the quest, and therefore I am unable to complete the mystery. I have tried reloading the save, I have tried leaving the area and coming back, and I have tried going away to do other missions and then coming back later. I have even tried carrying the dead body away to see if the character might re-spawn when I return, but alas this also did not fix the bug either.
I fear as though the only way to fix this bug would be to go back to a version of my save before I ever encountered it, but that save is long overwritten, as I have completed much more of the story arc since.
Area on the Map:
Dead body on arrival:
Any advice or help would be much appreciated.