Ohhh I have an idea!

You know how Ethan was going to message Cas about killing him? We can still have that, but perhaps they somehow find a way to message Iris. Threaten her, make her truly believe that they would bring Caspian down as they had spies on the inside along those lines if she doesn’t return knowing that it would work on getting her to go to them. 

This could then incorporate that idea I had a while ago. 

The one where Iris asks Jacob to take her back to the districts in secret. Spouting nonsense to him that she is not good enough for Cas, she will be his downfall, he deserves better and she should just disappear. Perhaps this is where he takes her back? Not realising she has done it because of the threats to Caspians life because they do the whole you tell anyone we will kill him and Maisie will be next maybe?