It's possible that Namshiel is more of a lone wolf who just happened to be working with Nicodemus during the events of Small Favor. He wasn't around when Nicodemus was working on the plague curse, so it's possible that Namshiel has his own agenda. If Namshiel was set up by either Tessa or Rosa to take the fall by agreeing to launch an attack on Arctis Tor, he's probably not happy with either of them. He probably didn't like being left behind on Demonreach either. While it was Namshiel's host who paid the ultimate price, I bet the Fallen don't like being sent into darkness because they never know how long that condition will last. So Thorny is probably no one's friend.
I would also guess that after the events of Skin Game Nicodemus must have come to realize that he was set up by Mab, though initially he might not have realized that Marcone was acting as a full partner with Mab, but I'm sure he knows now. Perhaps Namshiel has some way of blocking Anduriel's scrying ability, but even if he does I doubt he could keep it up 24 / 7. So Nicodemus probably knows that Marcone has Namshiel's coin.
Taking about who is driving and who is in the passenger seat, in upcoming stories it will interesting to see if Marcone's attitude, language or behavior seems to be changing; even if that change is a subtle one.