I suppose too as part of our discussion we ought to consider the possibility too that Raphael knew of the true likeness of Epicurus, but still chose to portray him as a wreathed cherubic figure as a means of insulting him.
That may be unlikely, but it is one possibility that if we're being complete we need to include in the analysis. Presumably the most likely scenario is that Raphael intended to portray Epicurus accurately, and the question is whether he had means of knowing the true likeness. Short of finding some document by Raphael himself stating his intentions, however, we can't presume to a certainty that just because a correct image was available in the early 1500's that Raphael used it. The full list of possibilities would include (1) A correct image might have existed and he chose not to use it, or (2) a correct image existed but he was unaware of it, or (3) no correct image existed in the 1500's except underground.