It sounds like you and C are on solid ground. You're right: of course you need to talk to her. And to him, about your concerns and how the three of you can address these issues sooner rather than later.
I'm concerned about M, though. Why is she breaking her own personal rules about being involved with a married man? If she doesn't want to be polyamorous, why is she continuing in this poly relationship? It sounds lke she's saying one thing and doing another, which is a big waving red flag.
Good luck to you all.