I just don't think Halbrand is Sauron. For one, Sauron forges the rings as Annatar and was mistrusted by Galadriel and Gil-galad. I think this plot point is key to show the flaws of Celebrimbor. Plus Sauron corrupts the Numenoreans as Sauron, which is key to show the hubris and evil of Ar-Pharazon. It seems more likely to me that he is one of the Nine, if not the Witch King.