A. WHM / CNJ can out damage DD BLM on stone weak monsters (wish I had a parser back when my friend and I were testing). On normal monsters WHM is only a little behind its nothing like watching a Fisher try to out damage a class - having all the survival skills greatly helps the WHM in comparison though. Difference between CNJ DD and WHM DD is just RNG.
B. WHM has GLD and PUG the only two sub classes you really need - you cant combo with THM's elemental skills anyways so unless you are trying to fight frogs or something just stick with stone and aero.
C. WHM absorbs all of CNJ skills
A+B+C WHM > CNJ
If you need DD gear you can just materia it, just like the BLM does.