Proyas the Place is the important part. He is certainly extremely important during the timespace of TGO, even in PoN. What we are saying is extremely similar, but I'd phrase it as such: The Kellhus made people special by choosing them, not the other way around. I think your definition is closer to "the author made the character, so its therefore specials" which, while true, doesn't satisfy my need for in-world explanations.
As Madness says, he might have been groomed before we even saw him on screen by Maithanet/Moenghus specifically for Kellhus. That's something like 30+ years of conditioning directly and indirectly by Dunyain. He's a well shaped tool, but uncoupled from the dunyain who knows what he might have been.
Also, Achamian isn't really special either. He's the Mandate-Place that was stepped upon to gain the Gnosis. The Prince-Place was fulfilled by Proyas, but could have been any number of Princes that had functionally similar experiences - which is what I was attempting to demonstrate above.
The point is this :They are all parts of a plan that gets from point A to point B (TTT). The Places (people) in between are inconsequential in the specifics, but extremely vital in their generalities. Kellhus needed to be taught the gnosis, he needed a prince to back him, he needed later a queen to bare his children, good generals to lead his war efforts to groom to be his exalt-generals, and eventual general-in-fact while he was gone. He needed many different positioned filled, but they are all just pieces on the benjuka plate. When he moves them, the rules change - the pieces just do what they are told.
I'm thinking that it is important to keep in mind the contingent aspect of Kellhus' quest. He has to use the tools
at hand. As Kellhus is introduced to the Earwan ruling class, he chooses Proyas as a result of his scrutiny, the Probability Trance, and his search for the Shortest Path. His use of Cnauir was contingent, and the reverberations of that choice may still haunt Kellhus.
I believe that Kellhus chooses Proyas because he is the
best tool for a certain project. So in that sense, he is special (and perhaps irreplaceable).