A theory about what happened
Parents killed their daughter in a fit of rage. They were planning to dispose of the servant's body once the case had cooled down a bit. They were expecting to keep the police busy in searching for the absconding servant and in the meanwhile dump the servant's body somewhere far away. They locked the terrace door and covered the viewing area with a sheet and the body with a "cooler panel".
The parents could not dispose the servant's body in the night time because the security guards would get suspicious about their movements in the middle of the night. They did not plan to dispose both the daughter's and the servant's body because it would be harder to explain the missing daughter whereas they could blame the daughter's murder on the missing servant.
The parents bribed some of the policemen to tamper with the evidence but were not very successful as this became a high profile case and the inspector-general himself took the charge of it and there were more officers involved than the parents could approach for bribing.
The parents then found some connection in the CBI who promised to help them for a massively large sum of money. This CBI officer took charge, who is played by Irrfan Khan in the movie, and he moved the suspicion away from the parents and instead came up with the plan to implicate three servant class people using money and coercion.
Sadly the servants in India are so poor that they can potentially be coerced into accepting even a murder charge for a relatively large sum of money that will go to their family, with the fake promise that they will be taken care of and at the worst will not be in prison for more than a year or two.
The CBI is not so corrupt that any given officer would have done something like this, however this was possible with the particular officer played by Irrfan Khan, he had a reputation for it, he is actually the same officer who after taking charge of the Nithari serial murders case discharged the accused businessman and instead implicated his servant successfully, and who was also in charge of the controversial Rizwanur Rehman case which he closed as a case of suicide freeing the accused industrialist family.
The CBI heads also suspected this particular officer of corruption and asked him to retire from service and this case was given to an officer with a better reputation who successfully brought the case against the victim's parents. This new officer was not corrupt and the parents also by this time were out of money after giving crores of rupees in bribe, all they had and more, to the previous officer.