It is a bit of a conundrum; the known White Council could never have won the war with the Reds in the way the Merlin hinted, and the known White Council could not arrange things so that Harry had his opportunity.
Langtry might have been planning to use some other supernatural WMD, but Ariana beat him to the punch with the bioweapon than disabled most of the Edinburgh complex during Changes.
Maybe? Up to this point I don't think he's had Harry do anything though.
Well, there was the "try to clear Morgan, but you're on your own if you can't bring in hard evidence" bit. But Harry sort of put himself forward for that anyway.
Or Cowl is Langtry and the purpose of the Bloodline curse was to kill Ebeneezer and Harry. Leaving him free to use the Dark Hallow to become a god at which point, the Reds would have been toast and the White Council would become neutered. The Black Council then becomes the new White Council. With Langtry as God King. Improbable but fun to think about. I assume JB will reveal the plot eventually.
The scene where Cowl and Harry fought outside Bock's over the Erlking book is pretty important to evaluating the Cowl-as-Langtry theory. Specifically, where Cowl says he was curious to see how Harry actually stacked up in a fight, and what the Wardens were so nervous about. It could work - Langtry would certainly have access to the Warden files on Harry, but hasn't seen him directly do much.