Unfortunately, you can do nothing about your H speaking to you through your D18 at this time.
It is not right and the worst thing you can do is stop taking info from D18 as that may cause severe alienation for her. It already will be making things bad for your D18. Your H obviously knows the circumstances between you and D18 and he's likely wedging things deeper by speaking through her so she empathizes and agrees with his POV.
Please try to understand that he has his reasons. It serves him. He is likely acting out of fear. Do you have any idea what that might be? Understand that fear sometimes shows up as anger, so what is he "angry" about that he talks through D18?
I don't have any real thoughts right now about how you might go about doing this. I would recommend digging really deep and determining how you can speak with him directly.
There was a time when my W did this with my kids. What I started doing was, I would take the info and then I would respond directly to my W. Over time, she eventually stopped speaking through the kids.
