Great topic. Some suggestions though:
I) I'd use swingweight instead of plow through, as plow through is a function of swingweight, weight, and balance. Adding lead at the throat has more noticeable effect on plow through than it has on swingweight.
II)B) Adding lead below your hand, ie at the buttcap, does increase swingweight, though very little as the distance to the 10cm axis is very small. Same goes for adding lead at the throat. What you're saying isn't wrong, but it can sound like adding lead below your hand doesn't effect swingweight because it's below the hand.
Final note) The specific amount required at each position will very (hopefully only slightly), I suppose you mean vary?
Does twu have an article with definitions of power, plowthrough, swingweight, etc? Could be useful to copy them here so everybody knows what those terms mean.