I think the argument for Harry being the Merlin's ace-in-the-hole is primarily Doylist. From Jim's perspective, the reason to build a book around a concept like a summit is to play with Harry's now divided loyalties.
Very logical. But in order for that play to make sense, the Merlin's agenda has to have changed. His old agenda was to return to the status quo. I can see no scenario where the Merlin would rely on Harry to achieve that outcome.
The WCW's inability to defend all of this now-exposed territory is why I feel like the Merlin might be willing to trade away the safety of some portion of the vanilla mortals and lower talents for peace.
Of course the Merlin would trade away territory. Giving away territory is easy - all the troublesome nations would happily accept the territory of the Red as a gift. The peace talks would only be challenging if the Merlin did NOT want to give away most of the territory. And given this territory might strengthen very dangerous potential enemies of the White Council, the Merlin has a strong incentive to manage the territory changes that come out of the Peace Talks.
* The White Council may lack the strength to keep large blocks of the territory
* The White Council may lack the enthusiasm/support to keep large blocks of the territory
* Other powers may object to the White Council claiming the territory.
If the purpose of the White Council is to leave the Peace Talks with some agreement that aids the White Council in keeping a lot of this territory - or at least make sure that the territorial gains other powers make do not threaten the White Council, that can be a very good use of Peace Talks (in the eyes of the Merlin).
I didn't state that Harry would be a knowing and willing ace-in-the-hole plan.
I did not make that assumption. But the Merlin "knows" Harry to some extent and would predict the likely outcome if Harry gets involved (insults, threats, fights, disruption, etc). The only reason I can think of for the Merlin to want harry there is
a) he wants the talks to fail and the war continue
b) he expects the talks to break down into open fighting and he wants one of his most dangerous wardens there in the fight
Besides, I thought I read on this site that Mab was the one insisting Harry be at the peace talks - against the Merlin's wishes.