Marcone has plenty of other ways to get muscle, and he is a pragmatist. If he wanted to disrupt the peace talks, there are plenty of more mortal ways to do it. Like bombs and bullets. If he really wanted a monster, why pick an Outsider? Whole menagerie of Norse monsters, not to mention all the other types that I am sure Monoc could help with. Why pick something so dangerous and unpredictable (of which information is scant)? Not to mention (as has previously been pointed out) that Vadderung wouldn't stand for it. Not to mention freaking Mab, who literally last book worked with Marcone.
Now it could be Cowl, but it doesn't have to be. Just as likely him as not. And just because a Wizard doesn't use black magic, doesn't mean he can't. Harry's magic is tainted but it doesn't stop him being on the Council. Freaking Eb has used black magic by those standards, and he is on the Senior Council! Cowl's magic does not reek. It merely is tainted, much like Harry and Eb and probably Molly. He could very well be still hiding and an active member on the Council. The White Council is so arrogant they couldn't imagine anyone had infiltrated them, and look how far Peabody got. Not to mention how many other agents are likely on the Council. Cristos comes to mind, and he certainly has some shady support (which if you cast your mind back, apprehended younger Wardens at the end of Changes). I think we are about to learn a lot about what has happened to the White Council in the time between Changes and now. And I reckon Cristos will be more powerful and influential than ever.
It could also just be another Cowl acolyte, or another "Black Council" operative. That is just as likely as not.
It could be those same humans that aided the Red Court during the war. Almost certainly the same group.
And for those that were wondering about the Fomor-Outsider connection, I did post a WAG a while back but I might post an updated version in light of some new information and thoughts. But what I will say for now is that there is a clear link that these groups are all being pushed around from behind the scenes, and the Fomor seem to have taken the place of the "face" of the enemy whilst the real foe stays in the shadows. And I think the Fomor have been linked with the strangeness in the supernatural world of recent times, and every time things get weird (for the supernatural world) Outsiders always seem to be involved.