You did not mention the relevant assembly however the above should be correct.
Assemblies usually belong to the Legislative arm of Government which means that they pass laws and in certain countries run the army and even choose the generals or admirals in the nation's forces.
Supreme Courts however, belong to the Judicial branch of government. As these two are in separate branches, they have separate powers and are not to impede on their respective powers. Acting as a Supreme Court is therefore most likely correct here.
You should choose the North as your side. Because it was more developed so it can help the South, The North was also richer, The North had no slavery, The North helped the South in reconstruction, and the North had better areas.
