The only reason the special order scenario is so popular for CSA victory is that it was still early in the war before the Emancipation Proclamtion (the E.P.) and therefore while foreign support was still tangible.
Plus with the war so young, the Union's greater reserves of manpower and resources had not been fully tapped, as well that the popular opinion was still lukewarm following the string of defeats dealt by the CSA.
The only reason Antietam ever occured was because McClellan moved to intercept Lee after learning of his march orders from the 191 dispatch. Had McClellan never found the orders, it is likey that Lee could have slipped behind him and threatened D.C. or Philly, Baltimore, etc.
The question would be whether Lincoln forced McClellan to engage and displace the rebels (which no doubt Lee wanted very much so he could choose favorable ground) or allowed McClellan to engage at his own discretion...unlikey.
The politcal fallout from OTL Antietam was that Lincoln felt confident enough to