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?
Most of the leaders of the Confederacy should have been allowed to take a loyalty oath and most of them should have been barred from ever holding a government office ever again. This woudl have kept them out of power and also put their brainpower into the private sector of the south.
Freedmen should ahve been given homesteads in the west if htey wanted it or a cash equivalent. This should have been paid for from the siezure of property from Confederates who would not take a loyalty oath and by the sale of all plantations to individuals. This money woudl have also paid the missing back taxes.