Does the Hadith about the marriage of Safiyyah (may Allah be pleased with her) mean that the hijab is exclusive to free women (the Mothers of the Believers) and not to bondswomen, and was this before or after the verse of hijab?

The discourse that followed the imposition of the hijab on believing women, not on the slave woman, was that she should not cover her face, as Umar (may Allah be pleased with him) used to prevent them from doing so. An exception to this is the presence of temptation (fitnah), in which case they must do what prevents it.