One question that remains unanswered is whether all of the Fallen line up with Nicodemus and Anduriel on how to best deal with the Outsiders. I've been thinking that perhaps some of the Fallen might believe they have alternatives to a scorched earth policy that Nicodemus and Anduriel might be planning. Similar to Cowl, Kumori and Corpse Taker talking about getting prepared to take advantage of a New Order that was about to arise, perhaps Namshiel or some of the other Fallen believe they can work with the Outsiders for their own ends. Until this issue is clarified, I think it's difficult to know what Thorned Namshiel's long-term game is, other than further corrupting Marcone and finding a way to get in the driver's seat of that relationship.
I would also like to know what, if anything, Marcone has learned about the Outsiders. It would indicate how much information Namshiel is sharing with Marcone.
As far as Marcone's decision to take up the coin goes, I think that after Gard's plan for a safe room within a safe building failed and Marcone was taken prisoner by Nicodemus, he felt that after all his precautions had failed that he needed something extra he could pull out of a hat. He saw the chance to pick-pocket Michael and he took it. We can't be certain when he went full Knight of the Blackened Denarians. I would guess it wasn't right away. Perhaps after Harry came back from the dead or before that, shortly after the Fomor started making their presence felt.
I think I need to reread the short story Even Hand and see if there are any subtle clues that I wouldn't have noticed the first time I read it.