Religious grounds
Surely under canon law, a civil marriage is not valid anyway. So regardless of whether Mrs Simpson had been married before and regardless of whether Mrs Simpson had not annuled her marriages but divorced, then surely Mrs. Simpson was not validly married under canon law. Therefore, any religious marriage she would later partake in would be a valid marriage under canon law and would make her eligable to become Queen. The political implications (given that civil law is a legal marriage within the state) is a different matter but as she was divorced under civil law there surely was no hindrance to any subsequent remarriage.