I'm not convinced about this conning business. Please explain what it is that the co-pilot can perceive about the aircraft's position in relation to the landing area that the flying pilot can't? Isn't the landing area painted with a big circle and a "H" and maybe even a diagonal, straight, and forward limit (cross deck) lineto help the pilot with his positioning? I would suggest that if you are depending on a co-pilot and/or marshaller for a "good" landing you shouldn't be there in the first place.
And as for taking the marshaller's signals as mandatory - no way (apart from "wave off").