For the view of Epeirotes as barbaroi, we can thank/blame the eighth and seventh century Greek colonists. When they arrived with their prior colonial experience and maritime focus and polis centered life, they branded the Epeirotes as barbarians. Maybe blame Thucydides in this too - his claim that the Aitolians armed themselves "just like barbarians" (1.5.3-6.2) influenced other people. The Aitolians, he said, were similar to the "Ozoloan Locrians, the Acarnanians, and those in the continental lands (i.e., Epeiros)". But Thucydides isn't making stuff up entirely. The reasons for his viewpoint probably had a lot to do with the colonists who settled near the Aitolians and (by sea) related to other peoples their encounters with people who certainly didn't see eye-to-eye with them. He sees things from the point of view of the people of Ambracia and Amphilochian Argos, the inhabitants of these colonies. And it's very interesting to note that he says that the people of Amphilochian Argos which really was not a colony, but a much older city with a Trojan war era foundation story, needed citizens, and asked some of the people of Ambracia (which was a more recent colony) to come in and join them. Thucydides says that "adopting the Ambracians' language, they became Hellenes". It was not the blood but the change of language and colonization that mattered to him. Thucydides would have classified the non-Hellenized Amphilochians as barbarians, but those with the culture and language suddenly are Hellenes. So does it even matter what Thucydides thought anyway? What did the people of Epeiros think? They believed the nostoi stories that Pyrrhus/Neoptolemos was the ancestor of their ancient (Molossian) kings.

I'm not saying that there are any simple answers. Or that I have them, but neither way is it clear cut and either way it all depends on the point of view of whoever seems to be talking at the moment. If people want to go by blood or by customs or by whatever, they're likely to get different answers in different places. If someone asks me were the Epeirotes greek, I'll just say "sorta" and smile. It will invariably lead to a "why?", which will then lead to a much more accurate and interesting discussion of the nuances, and the more folks know, the better.