In your case it is PM report which is relevant. Violent asphyxia death leaves ligature marks and other signs such as blotted eyes etc. Forced constriction of wind pipe and asphyxia due to fluid in air passage have different forensic pathology If the PM establishes homicidal death then only last seen theory becomes relevant. If Police has registered a case of murder, there shall be something incriminating in the report. You take help of lawyers who are expert in medico-legal cases. As such a general advice on criminal procedure would be of no help to you