Provocative research. Obviously the cannon-balls were THERE at the scene to be scattered on the road.
The cannon-balls would have been randomly scattered on the road in midst of battle, but right after things calmed down they would have been moved aside by people who wanted to use the road.
I think Fenton re-created the scene he saw before the road was cleared (or if he didn't see it first-hand, it might have been as it was described to him by eyewitnesses, or he could have seen battles like it before and known what the scene would look like).
So is it wrong to make a scene look like it would have appeared, when you come upon a scene that was recently altered?