To clarify, Marcone was not the first mortal Signatory. He was the first mortal Freeholding Lord.
My personal vote would be that the Venatori are, but the Fellowship is not. The former has more of an agenda than just fighting, and I could very much see them as Signatories. However, I could play devil's advocate and say that they cannot be, as they are fighting the Red Court without repercussions from the accords for an unlawfully declared war.
Really, whichever fits your story better should be the answer.