Another perhaps small point I would ask them is IF the reference to elders in Jerusalem being the first century governing body as the basis for the eight JW governing body today, why did the entire congregation in Jerusalem have authority to make a decision and who exactly does THAT apply to today?
(Acts 15:22) . . .Then the apostles and the elders, together with the whole congregation, decided to send chosen men from among them to Antioch, along with Paul and Barʹna·bas; they sent Judas who was called Barʹsab·bas and Silas, who were leading men among the brothers. . .