"Could it be that as ~524mm is about the distance from elbow to tip of middle finger in an idealized forearm..."

524mm is not the length of the forearm. The length of the forearm is about 450mm or 6/7 cubit. I don't think the cubit was derived from the length of the forearm, and I don't understand why the forearm would be used as the sign for cubit. I think it was derived from the threefold division of the canonical height (ground to hairline). Ground to kneeline is one cubit; kneeline to elbowline is one cubit; elbowline to hairline is one cubit.