Gambetti's witness
Gambetti represents the reader, because the prose is close to Murau's point of view, so like Gambetti, the novelist shows what Murau wants to believe about himself before examining that belief further. Gambetti knows this about Murau's family: he knows they are allegedly artistic philistines, that they hide behind the glamour of their estate, and that their Austrian pride isn't all its cracked up to be—Murau says that his family has some unsavory political secrets. That is the story Murau believes about his family, and the reader has no reason to distrust him, so we are like Gambetti, learning these stories without seeing the characters up close.
Uncle Georg
Georg is a "good" member of the family to Murau. Namely, he is good because he shows that Murau is not insane or dangerous for leaving his family behind to live abroad. This complex character symbol represents the way that Murau longs to be separate from his family, but still depends on his family to show him how to do that. In this symbolic relationship, the reader might begin to sense the duplicity of Murau's feelings about his family. If he hates the family, what is the benefit of family allies? He might as well leave forever and never come back—which he swears to do. Then, when his parents die, he goes back on his word to return for their funeral and to inherit the estate he swore he'd never set foot in again.
Life in Italy
This is a subtle use of symbolism. If Murau is right at all about his parents' sense of snobbish aristocracy (which he says is unfounded; they are secretly not that refined he says), then his decision to claim Italy as his home, telling people he feels "thoroughly Italian," is a dig against his parents nationalistic pride. There is a symbolic implication in his decision to go celebrate art in Italy instead of enjoying his Austrian pride.
The death of his parents
For Franz-Josef Murau, family is a frustrating and bitter pill to swallow. He doesn't actually hate his parents. He looks down on them, but it isn't until they die that the reader understands the truth about his resent for them. The death adds a layer to his perception of them that clarifies (at least for the reader) what his deal really is. By celebrating the timeless arts, the immutable qualities of true genius, etc., he is kicking back against his animal nature and mortality. When they die, he comes to a moment when he admits that (just like them and their artificial identities as aristocrats), he has judged them from an artifice of his own. Secretly, it is fear of mortality that made him wish they were better. They were so darn human, and to be human is to be an animal, and to be an animal is to die. This is why this death symbol is implied in the title; the word Extinction speaks to animal mortality.
The estate
This is another symbol that would be easier to appreciate if the European model of life were better known among popular readers. This estate is a valuable possession. Often, estates were passed down for generations and generations, and one's status was tied up in the estate. With rights to own an estate, one could be said to be aristocratic (much to Murau's chagrin). When he gives his estate back to the Jewish community (that his family had spurned in years past), he denies his own right to inherit the power of his patriarch. It is him sticking to his guns.
