The southern compromise proposals floated by Crittenden, Toombs, Davis, and Hindman were all proposed in January 1861.
A couple of problems with this arguement that I see. First, it means that the southern cause was built on a lie. Second, if it was about tariffs then I would see that as a more universal rallying cry than slavery. We're constantly told that only a minority owned slaves but taxation would hit everyone, wouldn't it? Third, tariffs were not a problem. The south paid very little of the tariff income to begin with.