The latest, and cleanest, PoD for a CSA victory I know is the Lost Orders staying lost leading to a successful Maryland Campaign; being prior to the Emancipation Proclamation, anything later would make it several degrees more difficult for Britain (and by extension France) from offering their "mediation", which would be crucial to any kind of CSA Victory. (It also doesn't help said campaign would be less than two months prior to the crucial midterms.)
(Oh, and when James McPherson did a "What if" essay, he chose mentioned PoD -- just wanted to mention that...)