I think Tripti is responsible for karan nandini death. Look at it this way. Tripti was talking to a mystery man. Also since karan nandini are dying, ekta will try to make up by bringing ganga back into the house. Ganga can only come back if tripti goes!
When it comes to Tanya, as others have said, I feel she won't kill them but after their death will misuse karan's money and property.
Well ekta, all we've got to say is, you're losing both your brains and money!