Even if gets to the point where the Kentucky Supreme Court rules against the Republicans and the Senate, they don't have to seat the Democrat. They can still opt to declare the seat vacant, since the winner was ostensibly ruled ineligible by the courts. The seat would then be filled subject to the provisions of Kentucky's constitution (appointment? special election?)
Kentucky requires a special election for vacant seats. By the way, just to note, after January the Kentucky Supreme Court will have a makeup of 5 Democrats and 2 Republicans. Chief Justice is Republican though.