Just started a re-read of TGO and trying to fathom Kellhus' motives with his re-working of Proyas throughout the book (the head on a pole still makes no more sense to me than before).
Kellhus needs to retrain Proyas' faith in order for him to do something that he would otherwise never do. First, he de-humanises the God (It rather than He) and then "villainises" the God (our damnation is the God's harvest). He also completely breaks Proyas down by admitting that he (Kellhus) is a fraud and everything Akka said he was.
Why? Why does he do this?
He does not need to re-program Proyas if his only goal is the destruction of Golgotterath and the Consult. Proyas would already be more than happy to do this. What does he need from him that a) Proyas would otherwise never have done, and b) serves Kellhus' ultimate aim (whatever that is), perhaps in his absence?
So, I think Proyas will be the one to "release" or at least allow the release of the NG. What else could it be?
Kellhus needs the NG to close off the outside (I also believe either he or Kayutas, probably the latter due to the symmetries with Nau-Cayuti, will become the NG). Proyas will act to bring it about. I can't think of any other task that would be so anathema to Proyas and require him to be broken down so severely.