If the south hadn’t even seceded, I guarantee you you would still have had those laws happen. The biggest difference would have been the south kept their slaves longer if they never seceded.
Being too weak on human rights was the problem for Jim Crow, not being too weak on rebellion. The hypothetical occupiying armies would have been there to squash rebellion, not try to further elevate rights of the black population.
While more equal rights might have been a more prevailing sentiment in the free states, they didn’t have the will to stand up for those equal rights in states hundreds of miles away. They might have managed to abolish slavery in 12 or so years federally if left to their own devices, but they weren’t going to police the nuance of citizens’ rights in states they knew so little about.