I don't understand M's part in this, frankly. I don't understand why she is invested in the relationship between you and C - are you all part of the same friendship group?

It's annoying if people tell you you misunderstood when there was clear body language that you haven't, and yes he's being immature and yes he should tell you himself what he wants. I'd forget he ever existed, to be honest. Good on you for not doing anything behind his girlfriend's back too.