One clarification required from your side. What makes you to tell that they sold the gold? Did they informed you that? Also it is quite unbelievable that your father just returned the entire ornaments to them on their demand and never asked to return it even after 1.5 months.
You just think from their side. Are they making a valid claim? Any court will accept their claim. A photograph of her with all ornaments at wedding is a valid evidence. If they are able to prove the source of their income to purchase this gold, its more than enough.
What ever you are trying to discuss here as evidences against them seems not stand in a court as it is common scene by husband and his family to deny the acceptance of gold and money as patrimony. stamp paper value of property cannot be challenged because of many reasons (even officials say that notified valuation for the area is just 3 lks).
In case you are genuine on your side, try to settle it through mediation. Otherwise you may get into real trouble.