What does Tenri whisper to Diana?
Posting as answer because can't use spoilers in comments, still just a speculation...

With the recent development in the manga, it seems it might be something along the lines of:


  Keima is the one that chose me to be your host 


Also, remember Diana grew her wings once before (although they didn't notice), in manga Chapter 139 (can't remember which episode), after Keima apologizes to her in his library.