General Carroll,
Maybe what you need to do is back up a bit historically, rather than forward. If you are assuming the cigar orders were never found, would Little Mac have been audacious enough to have crossed the mountains and attack the Rebels at Sharpsburg? I think not.
What was Lee's original plan? Wasn't it to move further north, draw the Federal army out, and attack on a field of his choosing? He achieved the first two, but Sharpsburg was simply the most centrally-located rally point -- he didn't have much choice about the field. As it was, the Rebel army was barely concentrated when the Federals began (finally) attacking.
If the cigar orders had not been discovered, what would McClellan's probably course of action had been? The historians in the club are in a better position to say than I am, but I'd guess he would never have made it across the mountains. Many of Lee's stragglers would have had time to catch up after the hard march, so he would have been stronger. (Yes, I've read that many refused to leave their home soil, saying they had signed up for a defensive war and not an offensive one. I expect that was just an excuse for many of the foot-weary rebels to lay down for a while. Given a few more days, my bet is that many of them would have caught up with the army.)
I also don't know the larger geography, but I can envision Lee sending his cavalry east across the passes to locate the Federals and lure them back, stationing one corps to dig in and hold the mountain passes. The other corps would move further north and around (or over, depending on the distance) the mountains, then swing back down and hit the Federals from the north (the Yankee right flank) -- at which point the remaining corps would go on the offensive and attack. This was a favorit tactic of Lee's -- he had already used it successfully on the Penninsula and at Second Bull Run, and would later use it successfully at Chancellorsville and again at the Wilderness.
Another 'what-if' scenario that would favor the Rebs (if that's what you're looking for) -- Lee holds the mountain passes with his entire army (well dug in, of course, with artillery situated to command the valley to the east) and McClellan must take the passes. Can anyone say 'Fredericksburg' [:D] ? Again, the action would mainly take place to the east of the mountain range.
Your humble servant,
Gen 'Dee Dubya' Mallory
David W. Mallory
ACW - President & Cabinet Member
CCC - Ensign, Georgia Volunteers, Southern Regional Department, Colonial American Army