Parson is a Creator
as he is from the world that originated Erfworld; how Parson conceives of himself directly affects how he interacts with Erfworld (you see this in his swearing during the destruction of Ruthlessness; how Sylvia describes "Lord Parson with an idea..." being not dis-similar to the feel of an Arkentool; but the real clincher is here: http://www.erfworld.com/wiki/index.php/LIAB_Text_59
Parson is time and time again described as I kind of agent of Fate, but also as a force of nature. One GM is Erfworld; the other GM is himself (he most naturally conceives of himself as a Chief Warlord since that was essentially his heart's desire that was latched onto when bringing him to Erfworld).
So if third book is the final chapter (which I hope its not), then Parson will essentially be his own side; if it is not the final chapter, then I think we need to find a side where Parson will achieve the most growth.
I do not seriously entertain notions that Wanda would ever turn against Parson. Her faith in him is transmitted to her decrypted units. For those units which were extremely predisposed to not like Wanda/Parson, then their disposition towards Parson and his actions are at best lukewarm. But, for those units which have high faith/loyalty you can see just how much Parson means to Wanda. "He is level 20, he is level a million..." Sylvia thinks Parson can do anything, and I doubt that that is far off from Wanda's own personal understanding of Parson.
The most crucial question to answer is: "What would Parson do with his new-found knowledge of Charlie?" Should we still believe that Charlie is on the guaranteed path to destruction as he tells Maggie? Or do we think that Parson will find a reason to pity Charlie even though Charlie has tried to kill him numerous times and is probably the only person to come close to succeeding? Of course I think the story would be remiss without a villain capable of matching Parson to some extent, so the loss of Charlie as an opponent would be hard to recover from as a plot point.
The next major question to ask is: "Is there any side left capable of matching a GK + Parson?" I don't think a RCC2 + Charlescomm can do it since the loss at Jetstone (also whether you believe my edifice of Parson being a burgeoning Titan or not; he is canonically capable of being a caster of some kind), since Parson will be able to access the Magic Kingdom virtually unopposed.
GK versus Everyone else?