Actually, it might be a way to get Helen to turn on him, and help bring him and his organization down from within. For Harry, for Nicodemus, or another party. Cowl could probably manage it, or something dangerous or nasty, that looks good to a mundane. Maybe Drakul or a significant power among the Fae. Bring her back, or partway, show Helen, and then start making demands in exchange for her continued survival. Or take her to the hospital, show her the girl and then make an offer to restore her. And incidentally, degrade Marcone in her eyes. Ask her why he hasn't told her about this, what is his motivation and so on. It really doesn't reflect well on him if you don't learn about her in the same context Harry did - witnessing the degree of care he has for her, tied to his soulgazes of both Helen & John.

As for the likelihood of Nicodemus in particular doing that, I think the fact that Marcone has a coin is more than enough for now, given from what we see of how he operates. Once he's taken up a coin the usual motivation to bring him down or punish his defiance is obviated. Even if Namshiel is of Tessa's faction, Nicodemus and Anduriel definitely seem to have power over the others. He seemed to think he could command Harry just through the shadow of Lasciel he believed was still in his mind. So from his perspective, he's got Marcone hooked and he'll be reeled into the boat eventually. That's all the win Nicodemus really needs, especially keeping in mind that the end of it is basically damnation, and the longer Marcone is operating his criminal organization and carrying the coin, the worse his torment will be in that end. Nicodemus is absolutely depicted as a guy who can wait for longer term gains, and forgo personal satisfaction. He'll kill Marcone when and if Marcone presents a real and significant threat to something important on Nicodemus' plate and not before.