Personally, I'm not convinced that any of the figures represent Epicurus.
Bernard Frischer, researcher and archaeologist from Indiana University who specializes in Roman history wrote a book called The Sculpted Word: Epicureanism and Philosophical Recruitment in Ancient Greece in which he attests to the “magnetism” of Epicurus’ portrait. He devotes several hundred pages to exploring different cultural depictions of Epicurus throughout history based on available resources. On page 151 he makes an important point: “Before 1742, when the Epicurus-Metrodorus double herm with ancient identifying inscription […] was discovered beneath the new porch of S. Maria Maggiore in Rome (the herm is now in the Capitoline Museam), Epicurus’ true image was not known.”
Raphael’s intended audience would not have seen portraits or busts of Epicurus, and, even if they had, they would not have recognized the face of the bust to match that of Epicurus. Painting a contemplative, bearded Greek would not have been as Epicurus-esque as would have painting a chubby, smirking Roman (for example).
The four Vatican frescoes were certainly painted within an allegorical context as opposed to a historical one. This is evidenced by the anachronistic presence of philosophers spanning several centuries, several of whom were never active in Athens. The setting of this piece is purely symbolic and not in any way intended to be literal.
It's like "Jurassic Park", filled with dinosaurs from the Triassic and Cretaceous periods.
In an architectural context, the four frescoes in the Stanze di Raffaello in the Apostolic Palace in the Vatican were designed to reinforce the Christian narrative. Therein, the inclusion of “pagan” philosophers is not meant to contrast with the divinity of Christ; rather, it signifies a harmony between ancient philosophy and Christian theology.
Thus, the Church's favorite two philosophers, and their supporters (such as Socratics and Pythagoreans) are featured front-and-center. The idolization of Plato and Aristotle is overwhelming. They reinforce the brand. The choice to include Epicurus – at all – would have been antithetical to the function of the painting, taken symbolically. While Raphael may have made that choice anyway, it is thematically inconsistent.
Painting Epicureans in the Apostolic Palace in the first place is contextually inappropriate (It may even have been dangerous). Raphael's inclusion of Epicurus and/or Epicurean philosophers in the Apostolic Palace may have been akin to Diego Rivera painting Vladimir Lenin in the Rockefeller Building in the 30s.
If it were the case that we had some indication that Raphael was a closet Epicurean who subversively hid hedonist-sympathizing clues in his paintings throughout the years (...the way Dan Brown frames Leonardo in the Da Vinci code), then, in my mind, it would seem appropriate to include Epicurus.
However, I think that fiction unlikely. I think the following two possibilities are most probable: (1) Epicureanism is not represented in this painting because Epicureanism is thematically inconsistent with the artistic context, (2) "Epicurus" is represented by the anonymous, chubby, smiling Roman stereotype, writing in the front.
Either way, the relevance I see with this painting to Epicurean philosophy is the reflection of the Christian Church's marginalization of materialism over a millennia. Raphael neither provides us with a glimpse at Epicurus, nor of Epicureanism. What he provides us with is either commentary of ambivalence, that it was not necessary to depict Epicureans clearly, or absence, that omitting Epicurus was necessary.
In conclusion, I don't see any compelling reason for Raphael would have felt compelled to include the Epicurus and his Epicureans. He was not painting a record of Athenian teachers; he was painting a picture of philosophical pre-Christians.