well, I hate answering my own question, but I remembered I overlooked one also important aspect of the story when asking.
as Lieutenant of Barad-dur, Mouth of Sauron clearly was charged with dealing with internal business, and taking care of the matters in the Dark Tower.
However, I have just found some quotes that make me believe it was just an ordinary emissary, and not a Ringwraith:
Quote:
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Yet all his ordinary spies[size=-1] and emissaries could bring him no tidings. ~ UT, The Hunt for the Ring[/size]
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So, because these emissaries, such as the one he sent to Erebor, didn't manage to bring tidings of where the "thief" and the Ring were:
Quote:
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At length he resolved that no others[size=-1] would serve him in this case but his mightiest servants, the Ringwraiths, who had no will but his own... ~ibid[/size]
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By the time he took this decision, the emissary had already visited Erebor, so I doubt it could have been a Nazgul, because Sauron hadn't decided on using them yet.
And I have another reason that makes me think that the Nazgul never went to Erebor. Considering the story in the UT we can see what effect the presence of a Nazgul has, effects much more devastating then the words of the messenger at Erebor. People fled from their homes in Rohan when they went to Isengard, fear went before them and lingered behind them. These are clearly not the effects the messenger has.
The hearts of the Dwarf chieftains are heavy, but not because of the messenger himself, but because of the message. They knew all what the messenger was talking about, and they all knew the might of Sauron - that is in my opinion reason enough to have a heavy heart. This quote is in my opinion very useful:
Quote:
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We needed not the fell voice of the messenger to warn us that his words held both menace and deceit; for we knew already that the power that has re-entered Mordor has not changed, and ever it betrayed us of old. ~The Council of Elrond
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Also, had they passed that way, we should have had reports from the Elves as well, since Legolas was present.
As Rune very well points out, the fact he had a "fell voice" or "hiss of snakes" do not mean he was necessarily a Nazgul.
So my conclusion: this was just an "ordinary" emissary, that "[size=-1]could bring him no tidings."[/size]