The reason why Jefferson Davis never was put on trial for treason was that he would have argued, successfully, that the Constitution and the Declaration gave the states the implicit right to secede if the federal government overstepped its authority.
Having Lincoln win an election was overstepping authority?
No. The 1860 GOP campaign plank to halt the spread of slavery into the territories was, however, very clearly unconstitutional. That Lincoln was elected president made that unconstitutional campaign promise essentially a fait-accompli.
I would have encouraged the rapid industrialization of the South. Not only would that have alleviated the massive poverty resulting from the immediate release of the slaves (by 1860 the capital of southern plantations lay in the slaves themselves, and not the land or the crop), it would have weakend the social structures that made whites think of blacks as only bonded field labor in the first place.
As for the Confederate leaders, most (but not all, e.g. Nathan B Forrest) were reasonable and should have been given their citizenship after taking the loyalty oath and a probationary period. Even Jeff Davis's ex-slaves had no problem with a monument being built in his honour after his death in 1889.
