but yet, 150 years later, I still can't get my inlaws (who aren't even from the South) to see that major point
Have never been able to fathom this idea myself. If the war were not about slavery, and slavery were not so directly tied to the whole ideological conflict, why on earth would Lincoln have bothered making it a war goal in the first place, or why would the South have bothered to secede over the issue of slavery?
Seems glaringly obvious.
although a lot of Northerners wanted to execute Jeff Davis for treason postwar, I think Lincoln had the right idea when he hoped Davis would just flee the country
The way I've always seen it was that Jeff Davis alive was just a down on his luck former rebel who had a mixed reputation amongst his former constituents at best, Davis dead was a potential rallying symbol for the South and diehards who would not accept Reconstruction. In hindsight, the letting the South up easy policy may not have been the best in terms of enforcing the enfranchisement of the freedmen, but in terms of reunifying the nation, it was far more foresighted.
Executing the South's leaders would have been problematic on many levels.