οὐκοῦν εἰ μήτε οἱ σοφισταὶ μήτε οἱ αὐτοὶ καλοὶ κἀγαθοὶ ὄντες διδάσκαλοί εἰσι τοῦ πράγματος, here οἱ before αὐτοὶ is probably not an article but a demonstr pr, otherwise αὐτοὶ would be in a attrib position.
I don’t see that. The article substantizes αὐτοὶ καλοὶ κἀγαθοὶ ὄντες, with διδάσκολοι as the predicate. Normally the attributive αὐτός means “the same,” but context here seems to be the emphatic use, “those who themselves are good and noble…”