Dear enquirer,
1. File RTI in passport department to find out how she has taken away the child without obtaining no objectin from natural father. If you find any forgery on any such document, file a criminal case in the concerned police station and pursue the matter. If you face any hurdle there, you can file a case directly before the Metropolitan Magistrate.
2. Once a criminal case is registered, you get the summons served on her. If those are not effected, then you can move an application before the Passport authority to revoke her passport in view of the pending criminal case. If they will not respond (I am sure they will not respond to your request) , you can move the High Court in writ jurisdiction to give direction to the passport authority to revoke her passport. In such eventuality, there is a chance that she will come back to India.
3. Desertion happened 7 years back. Even though the father is the natural guardian of the child, for the first 5 years, the court generally grant the custody of the child to the mother. As such, it is quite natural for you to wait 5 to 7 years to seek custody of the child, as the welfare of the child of 5 years or so lies in the companionship of the mother. My view is that you have not slept over your right to seek the custody of the child. Now, you have the right to seek the custody of the child. So, move appropriate petition seeking the custody of the child. In this, move interim application for visitation rights of the child. You will certainly get the visitation rights of atleast one to two months in a year in the intervening period of the child custody case. At the end, I say, the act of your wife to keep the child below the age of 5 in the custody of her parents, while she was away in USA was illegal and was a great moment for you to seek permanent custody. But you missed that opportunity. Bygones are bygones. Now also you have a right and method to get some relief in Courts.
Wish you best of luck.