Okay. My reasoning goes like this.
Dev was acting as Dev in front of all the family members apart from Radhs. But when it came to Radhika he was sort of reserved. The way he asked forgiveness from Radhs was more like Abhay than Dev. And also the hug was sort of one sided for me (I may be wrong but that's how I felt). At the same time he knew all the things Dev should know, including the Manthra which only Dev Raj Purohith would know. It also seems like he has left to Delhi without informing Matharamji; which means that he came there according to his own desire. (Rajjan or Vibha might have played a part here but yet it was not Matharamji) It was he who came forward to help Radhika going against the promise he made to his so called mother-in-law. He also promises his mother, brothers and Dadi that he would never leave them again but no such promise to Radhs.
If he has not gained his memory he would not have been able to chant the Manthra plus would not have dared to break the promise he made to Matharamji. Also he would not have been able to relate to the Purohith family so well.
Now coming to the precap he claims that he wants to 'go back to his house' only in front of Radhs . Which means the other Purohith family members are unaware of his demand. So he would remain Dev for them. But for Radhs he would act to be Abahay as he needs to get back to Kanika and expose them without making Matharamji taking a drastic step to hurt Radhika. Being Dev if he requests to go back to the CN Radhika would never allow him to do so specially after getting him after a long time and also after she gets to know that he is inviting trouble for himself. Also he might have got to know how evil the CN can be and he must be knowing that even taking Radhs into his side and trying to expose them is way too dangerous and best is to keep Radhs in the dark for the moment. In this way he gets to be with his family as well as exposing the evil characters.
I might be wrong but that's how I felt after watching the episode.
Please do share your thoughts.