Well let me put my two cents and who I think killed Arjan.
Uma per your suggestion I watched the show again and you were right babaji did take Suraj upstairs with him to call baa to tell her cow face was trying to throw Santo out again.
Ok we are all picking the usaual suspect, but I'm going to think out the box and go with something I don't know if was ever done on any Ekta show's or indian TV. I watched Thursday and Friday show again and their was one family member the camera didn't do a zoom on. When Sanraj was cutting the cake they had this person standing between two women that's not part of the family and I find that very odd. I really hope I'm wrong cause it would really make me sick if that person is the killer, and also the person who told Santo about DM and Arjan was fighting
I think the killer is our laadli Birwa. Everytime their is a function Birwa is alway's next to her father or mother but this time she was no where in sight. When they did the group dance she wasn't their either but they show with Champli when baa was telling the family no one will say what happen to the police. Birwa is almost Arjan height is not too far fetched for her to stab him in the back, maybe his eye's was open cause he was in shock of who stab him.