You have been given the primary source evidence from 1765 to circa 1860 how canister was constructed and how it functioned. What you are apparently trying to do is to prove a negative (as in how canister didn't function when the piece was fired) and that is a standard historical fallacy in that you cannot prove a negative. Evidence cannot be negative in nature, that being a contradiction in itself. Seems to me that you have been given enough evidence from period artillerymen as well as artillerymen after the period using the same type of ammunition constructed similarly on how canister was constructed and functioned and want to either disregard it or contradict it negatively, which is your prerogative, but you are incorrect both in your assumptions and in your ignoring valid primary source evidence.
Another historical fallacy you seem to be employing is that of the presumptive proof in which you are submitting a hypothesis and then asking others to either prove it or disprove it which makes your 'presumption' invalid.
I submit that your analogy of using the horse to canister to be illogical and presents somewhat the fallacy of the 'perfect analogy' in which you are trying to submit a partial resemblance between two nonrelated issues (horses and canister) linking them with a common thread (photography) and submitting that there is an exact correspondence between the two. There is not a direct correspondnence in this false analogy and it actually makes no sense though it does sound good and paint an interesting picture.
I would suggest studying some basic gunnery and ballistics texts and taking a look at more manuals of the period. While the artillerymen of the period could not 'see' what was happening in the bore of the piece when fired and the canister was launched down range, they certainly tested the rounds and saw what happened as the round exited the bore and hit the target. Their conclusion that the tin canister ruptured in the gun tube is exactly correct and why the field piece had a shotgun effect with the canister round exiting the tube as separate iron balls in a cone-shaped mass. It was an anti-personnel round, partially desinged for battery defense and the purpose was to have it rupture in the bore so as to cause casualties as soon as possible. That is one of the reasons the round was made of sheet tin.