1)What should be done to get FIR at the earliest ?
If the police is reluctant to register the case or FIR, you may file a private complaint with judicial magistrate court under section 156(3) cr.p.c. seeking its direction to police to investigate and register FIR.
2)What should be done to get the girl arrested as soon as possible?
She cannot be arrested without a FIR registered against her.
3)Girl had some previous affair and was also taking depression pills which her family members hide from us.
What all sections(dharas) are applicable on girl and her family based on suicide and hiding truth facts?
Section 306 IPC will be applicable, by which: If any person commits suicide, whoever abets the commission of such suicide, shall be punished with imprisonment of either description for a term which may extend to ten years, and shall also be liable to fine.
4)It also came to our notice that the girl side might go for Anticipatory bail and quashing of FIR in high court.
What should be done to prevent both Anticipatory bail for girl and quashing of FIR?
You may file an intervene petition during bail hearing and record strong objection to grant of AB and also in the counter for quash you may strongly object to the petition.
5)Also girl side had given some cash ,jewellery ,how to return it ?
Let them ask for it then you can return it in the presence of witnesses of both the sides under an acknowledgement from her side.