Yes, it's definitely one of those things where you just have to trust the author and hang on, and Aaronovitch has built up a good amount of trust for me. But it doesn't stop me speculating wildly in all directions anyway :-).

Looking at it from Nightingale's perspective is interesting - now I'm wondering if he would have set Peter up to betray him as well. He does go behind Nightingale's back in Moon Over Soho, but Nightingale doesn't seem to really hold it against him, probably because they argue it out afterwards.

And oh, the possibility of entrapment by the Faceless Man is a good idea too. If he's got any sense he won't be trusting Lesley any further than he can throw her, even if he thinks he's bought her loyalty.

Lesley must have an exit plan somewhere. It's not much good to her if she has her face and health back, but can't go back to any of her old life, after all. Now that I think of it, this seems to me to be a strong argument for her being a double agent. Assuming getting her face back is her motive at all and it's not something completely different like the Faceless Man having another murderous chimera camped on her parents' doorstep. As you say, the possibilities are endless.