I think that southerners hadn't suddenly changed their view from blacks being an inferior race and property to suddenly being equals in race and as individuals overnight, should have been entirely foreseeable to Republicans pursuing policy, as it hadn't even happened in their constituents
These things are not mutually exclusive. It's entirely possible that Republicans and southern African Americans knew that white southerners would remain racist, and simultaneously, believed that the same federal government which beat the Confederacy on the battlefield could enforce racial egalitarianism in the South during peacetime.
Several historians have recounted that many black southern politicians were assassinated during Reconstruction. I don't think these men were deliberately suicidal.
Certainly, Republicans and southern African Americans miscalculated the outcome of attempted racial equality. But human history has perhaps millions of cases of miscalculations that led to sad results. These continue to this day, all over the world.