Excellent questions and good points. All the evidence from the manuals, based on observations on canister firing, is that the canister either ruptured in the gun tube or at the muzzle. Because of the tin construction of the canister, I would say in the gun tube. The cone of the ballss forms from the muzzle.
I would also state that canister is a more efficient round than grapeshot, though the development of tiered grapeshot was an advance. However, grapeshot by the time of the Napoleonic period was used in larger caliber guns and at sea.
Canister was deadlier the closer it was fired to the gunline against advancing troops. What is being demonstrated is that it could be effective out to 800 yards, but if you read the manuals carefully, I don't believe that was the optimum range for canister employment. Balls at the bottom of the dispersal cone or at the end of the balls' flight could be expected, if the grouns was suitable, to richochet as round shot did.