Right about now perhaps the only way you could get "arbitrarily close to 0" is by scanning Knox's brain or the actual perpetrator's brain; if both should die, we would reach a state of absolute uncertainty about the event, all the evidence you could in principle examine to reach certainty will have been rearranged into unintelligible patterns.

This may be true if you measure from inside the universe, but certainly isn't if you can measure from outside, including observing other quantum branches. (Hey, you did say "in principle.")