Respectfully, pieces of this need correction. To wit ...
"During the Maryland campaign, Hill was mistakenly sent two copies of Lee's Special Orders No. 191, which detailed the divided positions of Confederate forces as it attempted to march north towards Pennsylvania."
Incorrect. There is no evidence that two copies of the orders were sent to Hill. Hill himself said he never received a second copy sent by army HQ, and he retained the copy sent to him by Jackson. It is available online in fact. Secondly, the ANV was no longer headed to Pennsylvania as of noon on Sept. 8. Toward it, yes. Into it, no. Reports that Federal troops still occupied Harpers Ferry and that the Army of the Potomac had begun marching toward Frederick stymied that part of Lee's plan. Even before the lost orders were lost, McClellan was moving faster than Lee found convenient. Whereas Lee had anticipated being able to operate on the Confederacy's "norther frontier" until winter, he now found that he needed to engage the Federals within days.
"One of these copies was accidentally left (likely never delivered) in a field near Frederick, Maryland wrapped around three cigars, where a Union soldier, Corporal Barton W. Mitchell of the 27th Indiana Volunteers (who would later be wounded at the subsequent Battle of Sharpsburg), discovered it on September 13th."
Correct - the copy was never delivered. This contradicts the previous statement about two copies sent to Hill.
"Realizing that Lee's divided army was now vulnerable, McClellan pursued the Confederates with uncharacteristic speed."
Incorrect. McClellan had marched portions of his army as far as 40 miles in order to put troops in between the ANV and Washington. McClellan then began advancing on September 8, while covering the route to Baltimore and DC, with some brigades making 15 miles per day. As of the morning on September 13, Lee knew McClellan had occupied Frederick City. What he did not know, because Jeb Stuart failed to tell him, is that McClellan had also pushed an entire army corps into Middletown Valley. D. H. Hill reported this after dark on that day.