Miles, from the looks of things is very shaken. He knows what Maeva is capable of, he even said himself he had “trauma” from their past relationship. Deep down he knows there is a big chance the baby is his. The meeting where Maeva consulted him before she decided to pop the question to James is a BIG giveaway that they are romantically involved. They then had sex that night and resulted in the pregnancy. The timings add up.
She would never run off with Miles properly, because he can’t offer stability but James can, therefore she stays with him to benefit from that, whilst Miles brings her the “passion” that James talked about. James isn’t strong enough for Maeva to lay down the law. Note how she said “Miles is my little witch I can get him to do what I want…” But really Miles doesn’t want to settle down, probably ever, that’s what she’s fighting with …It’s very fucked up.