I did a research project on Company "D" of the 1st Arkansas Infantry (CS) at the Battle of Shiloh. This company suffer close to 50% casualities according to their muster rolls. What amazed me was that the records in this particular case were remarkablely detailed. They included such details as the date and time when the soldier recieved his wound and what kind of wound he recieved and on what part of the body. I have never seen this kind of detail recorded at any other time or anywhere else. What really surprised me was that most of the wounded were recorded as being wounded at approximately 1:00 pm on April 6th and mostly in the LEFT SIDE.
This to me would indicate that the men were standing erect in line of battle and had recieved a volley from their enemy, while they were preparing to deliver a volley themselves. This would leave their left sides exposed to their enemy fire. Unforunately I can not remember how many of these wounded men survived their wounds to return to duty. But, it parallels what your Unionist writer was commenting on.
I can understand his opinion on the prone position because the first place most likely to be exposed to the enemies fire would be the head. A head wound would most certainly be fatal. But, I don't understand the kneeling position unless it has something to do with the vital organs being more compacted together in that position and a wound so struck would cause more damage to more organs.