Marie Antoinette's body was buried in a pit, not a tomb. After the revolution, her body was unearthed, identified, and then reburied in a coffin in the basement of St. Denis basilica.
Nuns are not trained as exorcists in the Catholic Church, so Carmen would not have known what to do to remove the evil from Augusto. However, as vampires are not real, it is not unreasonable that the world of the film did not also take poetic license into what nuns are trained to do.