That's correct and fine and well and good, but either the Earth is orbiting around where the Sun was centered 8.3 minutes ago at any given point in time, or it is orbiting around a point closer to the center of the Sun at the present time, and the differnece, if any, between those two points will enable us to calculate, from observable phenomena, the speed of Gravity.
Nope. The Earth and Sun are orbiting a center point of mass. Since the Sun has such a greater mass than the Earth, the point is inside the Sun itself. However, the Earth is moving along with the Sun in the direction of travel of the solar system.
A poor analogy, but should give you an idea, is you dropping a ball on a moving train. The ball falls strait down (relative to you) because it is moving horizontally at the same velocity the train is.