actually the starter relay is one of the few electrical pieces on the car that doesn'r require chassis ground. It only needs a positive and ground on the signal posts to energize the coil.
I'm leaning toward replace the starter. When everything except the starter is working properly the bendix drive solenoid will thunk ( engage ) but not run the starter motor. That doesn't explain why it works when you jump over the relay except that throws a little more current at it.
