Two brief remarks.
--> Philippus:
From a rhetorical angle, there is no point in formally quoting from a work (especially as "prophecy") unless the work as a whole is acknowledged as authoritative by the readers.
Or (ab absurdo): if the writers whose texts happened to be later retained by the "canon" and henceforth included in the "Bible" did have, in their own eyes and in their first readers' eyes, that kind of authority which automatically guarantees the truth of whatever they wrote, why would they bother arguing and quoting (i.e. referring to written authorities beside themselves) in the first place?
--> ms:
I think it is worth stepping back and pondering over the concept of mediation, and its relationship with ontological thinking (e.g. the notions of "essence" and "nature," as in "divine" vs. "human" or "created" vs. "uncreated"). In a sense, mediation dissolves in ontological thinking which in turn results in an endless inflation of mediations.
Let's say at some stage the priest-king is the "mediator" between a people and its relatively close deity (close in the sense that it is this people's god and not necessarily the highest god -- in a sense this god is itself part of the mediation to the divine realm as a whole). Now ontological thinking drives both sides apart. The priests, and kings, are thought of as "only men," and the gods (or "God") are too remote to act directly in the human sphere any longer. Then a new type of mediation appears -- in the Bible, "angels". But as soon as ontological thinking strikes again, angelic mediation is impossible too: "angels" are regarded as a third nature, neither divine nor human. Orthodox Christology avoids the problem theoretically by postulating the hypostatic union: Christ is both divine and human (hence different from the angels which are neither). But practically (at least once the Gnostic solutions you referred to are ruled out) he becomes yet another category of being, unique of his "kind". People cannot immediately identify with him and new mediations to the mediator (!) are required (hierarchy, sacraments, saints, etc.).
The dumbest question you can ask about a bridge is which bank it belongs to... This imo sums much of the Trinitarian/Christological debate.