The Kade scene for mystery hunting requires:
first passing a 60 courage or 80 manipulation check ---- > correctly answering his questions --- knowing the correct reason the servant was killed (requires figuring out a certain persons plot against you) ----- > correctly guessing Kade's motivation ----- >correctly guessing the method of murder / and three other correct theories of what might have happened (wrong guesses will work against your total score here) --- then passing an interpersonal insight or persuasion check
if you have done all those then you unlock his confession scene
hope that helps <3