This is all very interesting. I would think that casualties by artillery (at least, by ball) would be pretty evenly dispersed throughout the battalion. As you point out, the square was comparatively compact, and the "bounce through" capability of even a 6 pound ball would hurt the rear companies as much as the front. The quote by the Leeke goes on, I think, to mention the ball strking four men in the front face, but bouncing over the rear face. It could just have easily been the other way around.

So I'd think that uneven casualty disbursement would mean exposure to musketry, as experienced by the 27th. But the 23rd didn't see much musketry; perhaps it was some fluke in terrain that sheltered the rear companies. As you mentioned, perhaps a precise map location can help explain things.

Best regards......Mark