At the current time, are the very oldest dinosaur fossils from South America?
Yes.
Does anyone expect older ones to be found elsewhere? (speculation, of course) Could S.A. be the radiating point from which they spread to the rest of the super-continent?
This question is wrong. Being part of the supercontinent, SA didn't exist in a biogeographic sense (...nor, in fact, any other). Perhaps there was an ecosystem, a climatic zone, where the dinosaurs originated and from where they spread, but any such region would contain only parts of SA as well as parts of the neighboring continents.