Sorry, I disagree. The Confederacy, if you read their articles of confederation, speeches by their leadership & articles of secession of the individual states, regarded slavery & its continuation as essential to their economy & way of life. Their plan was to extend slavery into the Caribbean & Central America. There is no way they were prepared to end it.

Had they done so, what happens to the four million or so freed blacks?

Lincoln worked hard to shepherd the 13th amendment through Congress. He knew, by the time of his death, that it would pass the states & become law. So he did end all slavery in the US.

Had secession gone through, via war or simply letting the Confederacy go, I strongly believe anarchy would have resulted. There was really no reason for the Confederate states to remain unified. E.g., Texas & Florida had interests & economies very different from Georgia & Alabama. Furthermore, once secession becomes viable, why would Maine seek to remain in a union with say California or Oregon?

Reconstruction lasted barely ten years. By the presidential election of 1876 most federal troops had already left the south. Given that the election was disputed the Compromise of 1877 gave Republicans the White House in exchange for removing the last federal troops from the south. Thus by 1877 reconstruction was over. Hard to see how it was vindictive.