It had long been speculated that Marcone had the coin. Mab even made a point to look at Marcone during the conversation about what weapons could hurt Ethniu. One thought that I have had is what if Marcone only took up the coin for the fight. If he had however been holding the coin in reserve for such a time it would be in character for him. He has the willpower to talk to and use an imprint just as harry did, without taking up the coin. There is no reason he couldn't have gained the necessary knowledge from Namshiel prior to picking up the coin. As a side note, it is also possible for the fallen to take direct action so long as their host "wills" it, meaning Marcone would not have to have the magical skills himself to use them. Just as Butters was using BOB to power artifacts the fallen could be used to fuel Marcone's magic.