Ok my suggestion was that one or more of the lesser Nazgûl are movers behind the curtain, and the future Mouth guy is an up-and-coming guy/Nazgul puppet/guy who strikes a deal with the Nazgûl. So, ‘the Heavy’ will never be found, as it really is Sauron himself, and Gandalf doesn’t find out about the Nazgûl except through hints which aren’t very clear to him (Gandalf).I’d say yes, that the Black Numenoreans have declared for Sauron and are behind the movements of whoever we put as the Heavy*. If Sauron is not directly involved with the Black Numenoreans, the entity manipulating things could be a Nazgul; if we so chose this could be the Witch-King of Angmar. If the Mouth of Sauron is the Heavy in this season, he could be sent by the Black Numenoreans to bring the Haradrim over to Sauron's side.
*TV Tropes term for the active, plot-driving villain, ie Darth Vader.