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First, you need to carefully analyze the save file to understand two things:
1) Is there a clear victory for one of the sides, regardless of the other side's actions (within reasonable limits)? If so, there's no need to complicate things; you can simply award the victory.
2) Is there potential for what Derbent is suspected of? In other words, does the optimal move differ when knowing about the cloak/amulet and when not knowing? If it doesn't differ, then again, there's no need to complicate things; you can suggest continuing from the last save.
And only if the previous two points are not sufficient to resolve the conflict can you start thinking about other solutions.