Let me propose an alternative. Given McClellan's ego, his PR talents (you have to get to MacArthur before you find someone who has less military talent and a better touch for PR), and his inability to see disagreements as anything other than betrayal, a victory at Antietam might be a very bad thing for the North.
While the South would be crippled, it is unlikely that they would surrender on the spot, and McClellan would be the logical person to lead the March on Richmond. If anyone could snatch defeat from the jaws of victory under such circumstances, it would be McClellan. From past behavior we can anticipate that McClellan would take a LONG time to get moving, and he would move very slowly...slowly enough possibly to give the south time to mount some sort of defence. Even if he was able to take Richmond, this wasn't Grant with his manic pursuit...it is likely that the traitor's govt would have escaped to the west, which was nowhere near collapse in 1862. It is POSSIBLE therefore, that continued CSA resistance could have occurred based upon the South minus VA. This would have been a horrible blow to the CSA, but it would still be a going concern, particularly if there wasn't an aggressive attack mounted against it.
Here is where I believe McClellan would do the worst damage. I think that if the circumstances I suggest above came to pass (and I concede that these are something of a long shot), McClellan would likely oppose any significant offensive action against the CSA. After all, he would now be the hero of Anteitam and Richmond, he would posess enormous political prestige, and the last thing that he would want is for anyone to make clear that he didn't finish the job by capturing what was left of the CSA and ending the war with a complete, rather than partial victory. Moreover, McClellan was a conservative democrat (boy are those days gone forever!...) who opposed, an absolute war. It is very likely that he would have agitated for a negotiated solution with the CSA, arguing that with their capital taken, their army defeated, and the leading general in chains (or dead?), it was time to turn away from the position of the radical republicans.
Not a certainty by any means, but worth a bit of speculation?