Is a woman held accountable and required to offer expiation if blood is discharged from her at night after she had purified from postpartum bleeding and had sexual intercourse with her husband?

The purity you observed on the eighth day is a valid purity, and you must perform ghusl (definition) and pray. There is no blame on the husband for conjugal relations after purity, and no expiation is required for intercourse because it is permissible. The blood that returns within the forty days is postpartum bleeding (nifas (definition)) according to the most preponderant opinion, and you should make up the prayers of the two days preceding the ghusl.