1. At the breakfast table he acts like he does not know cornflakes and oatmeal. But when he is hiding and the butler sees him there he says "Go get me toast and butter". So that shows he is acting like he does not even know basic things like cornflakes and oatmeal in front of the family to make Ameesha suspicious!
2. After the hug when Kajal cries remembering Dev and says to him "I will never stop loving Dev, just because he is dead does not mean love stops" and she goes away, just see how proud he is to hear that! It is Dev for sure.
Then he says to himself (in good Dev hindi, not tapori) "Love ... when it gets you once it never leaves you". How does Raju know that if he has never had that love? And why is he thinking in Dev's language when speaking to himself?
Then as if remembering he has to be Raju he adds "Rangeela, hope someone loves you like that. And why are your eyes moist?" and then again he says with a certain pride and sounding like Dev again "This Dev Pratap Singh is a lucky man".
See all his expressions - first as Dev, then briefly as Raju, then again as Dev - and especially he is thinking in Dev language all this not in Tapori language even in his own mind!
I am totally convinced after so many repeats of the video and closely watching every expression it is Dev and he is in full control and playing a very big naatak deliberately to make Ameesha suspicious!