1. I read the whole essay and the only allusion that she uses is Xanadu. Joan Didion writes, nobody lives at Xanadu meaning nobody lives in the paradise.
Allusion is a brief and indirect reference to a person, place, thing or idea of historical, cultural, literary or political significance. It does not tell in detail the thing, idea, place or person.
Xanadu is a mitical place. It is located in the north of Shangri La that was introduced in 1933 in the fictional novel Lost Horizon by author James Hilton
2.- The symbol that Joan Didion uses is that nobody lives at Xanadu that it is paradise on earth for her because New York was appealing to the author even though she was not well paid and didn't have the best food on the table.