I can't see the error in the diagram. From an engineering point of view, centrifugal force does exist because it opposes the centripetal force. Stated differently, if it were not for the centrifugal force, centripetal force would zap the blade tips into the propeller hub. Stated differently again, centrifugal force is the force exerted by the blades because of their momentum. The idea that centrifugal force does not exist, as I think you have stated, is because if the blade were separated, it would fly in the direction of delta Q rather than the direction of delta C. Where this idea falls down is that the moment the blade breaks off, the centripital force stops, and the moment that happens centrifugal force stops, and the blade stops accelerating. So, if this is what you were referring to, delta C in the diagram is perfectly correct.