And when would it have been acceptable to put the issue of slavery back on the table?
It was taken off the table as a Federal issue, by the Constitution, and the compromises that led to the Constitution. It never really ceased to be an issue at the State level, even after the Cotton Gin made it more profitable. It did, of course, become very awkward to discuss, once the Abolitionist fanaticism became a factor. No self-respecting Southerner wanted to be associated with being on the same side with those ranting pure hate against your State and your neighbors.
Had all Northerners who opposed slavery acted with the honorable intentions of a Webster, Southerners would have been able to consider reasonable proposals to phase out the system, without appearing to be turncoats.
But why are we off on this tangent? The point of this thread is that the Negroes who were loyal to the South have been ignored. Surely you will agree that it is an injustice to ignore loyalty, which is a virtue, whatever your opinion of any social system. As I pointed out earlier, Booker T. Washington cited that loyalty of the Southern Negro to the Old South, as a very strong reason why the Southern business community should hire Southern Negroes, rather than immigrants. It is a very significant fact of Southern History. See Booker T. Washington.