I believe the reason the number 3 man was trained to keep the vent hole covered during the entire operation, prior to loading and during the loading operation, was that in combat, the barrel would 'foul' over time with residue, making it nearly or completely impossible to run a dirty sponge down it. Any assumption that the thumb stall was only required during part of the loading process, automatically assumes that water was always available to artillery crews in sudden combat - ideal for sure, but a nearly impossible feat to accomplish.
I also question the theory that it was more important to keep the vent covered while cleaning out "cooling down" the barrel of a 'fired' gun, but not worrying about doing this, while pushing a bag of gunpowder and a live round down that same barrel. I suspect, as a number one man, I would be far more worried about that live round going off while I am pushing it down the barrel, than I would be if some residual powder ignited from the last round fired - blowing super hot air out the barrel. That is why the number one holds the sponge rammer in the fashion he does - in case some residual powder does ignite, it potentially won't take his arm off. On the other hand, if your hand is in front of the barrel when a round comes out - well it really doesn't matter how your held the rammer - everything is gone.
From the book Detailed Minutiae of Soldier Life
As to which position was the most 'dangerous', I would be interested in any statistics that have been published to say any one position was more dangerous than another. While it is true #1 and #2 would be 'closer' by a few feet to the enemy, #3 was stationary and in full frontal position to the enemy during the loading process, while the others were mobile or standing sideways to the front much of the time. A sniper would find the #3 man one of the "easier" targets, in my opinion, unless the sniper was located directly in front of the cannon, where the location of the #1 man, could offer #3 some cover.
As one of my ancestors was a #3 man in a Union 1st Missouri Artillery outfit, I read first hand accounts of artillery units during the war with special interest.
There was the manual way of firing, and there was the way crews actually did it in combat - under ideal conditions. Under less than ideal conditions, they adapted. Today we have the NPS way and the reenactor's way, both of which give us a general overview of how it was done, but both emphasis safety over the way it was taught during the war, and limit our ability to understand the 'short cuts' some crews would be forced to take under certain conditions and in certain battles. Read some of the books written by artillery men, from both sides, during the war and you will see these crews were men - not robots - and men will quickly learn that if a shortcut and a faster way of doing something is the safer way when someone is shooting at you - then a shortcut is born. Men in combat also quickly learn that patriotism may demand sacrifice, but does not always require suicide.
Thanks for posting the story Laura.