I don't see Little Mac quite THAT stupid, SF. It would cause him a lot of headaches. All he has to do is to declare it a state matter and let the Southern states reimpose it.
That doesn't work, either, as the Confederate government tried repeatedly to do this in the 1861-5 frame in terms of for instance keeping slaves from running off, and it didn't work, while McClellan promoting CS leaders *back* into power and giving them command of *new* troops to hunt down *ex-Union soldiers* is worse than having Northern troops engage in this kind of thing.
Even before Lincoln gave the Emancipation Proclamation he already stated any slave who helped the Union Army in any fashion (he was talking primarily of spies at the time) would be granted full US military protection permanently from being re-enslaved and he said it publicly enough that it would be difficult for him to go back on his word. So I don't see him doing so. I see him more not preventing them from re-enslaving non US Army personnel.
Even before Lincoln gave the Emancipation Proclamation he already stated any slave who helped the Union Army in any fashion (he was talking primarily of spies at the time) would be granted full US military protection permanently from being re-enslaved and he said it publicly enough that it would be difficult for him to go back on his word. So I don't see him doing so. I see him more not preventing them from re-enslaving non US Army personnel.
McClellan can certainly go back on Lincoln's promises, though. Which adds to the worse-than-OTL bad.
McClellan opposed the Proclamation and gave full rein to his subordinates proposing a coup against it, and even wrote the President a letter telling him the Proclamation was a bad idea. This guy's the archetypal Northern conservative, he will *not* care about black lives or freedom one way or the other.