I was just thinking, perhaps she was terminally ill or had a serious medical condition due to which she suffered. Her father alluded to it as well.
At first, I considered the possibility of a drug overdose but then I wondered if HS and she decided to relieve her of the pain and "help" her die sooner, i.e. they opted for euthanasia. She might have begged him to set her free as she could not bear it anymore.
This is why he says he killed her, because he did, technically but not with the intention to murder her. He says "usne praan tyaag diye". He also said that he had so much "moh" for her that it took her life, i.e. his love for her meant he could not bear to see her suffer like that. They had that kind of an insane, passionate love. She must have died in his arms in that case.
This would make everything that Sartaj Singh said about HS being guilty of culpable homicide actually be true, as well as Naeem Bi's stance on HS being innocent. He could not take the fact that his daughter had been shown the door to death sooner than it would have naturally happened, all he had was memories of her suffering in her last days. It would also mean that what HS says about him being responsible for her death is actually true, though not with malicious intent of course.
If this is indeed the case, then I would applaud the CVs for going this route in terms of her cause of death. Then again, I could be completely wrong and there may be other possibilities we could never have thought of. Whatever it is, I'm sure it will be fantastic.