Hi,
Hope you are doing well !
1. Yes, you can. Since rent is paid to owners, the property must be owned by your grand parents. It may be owned by one or both of your grand parents. Remember that you cannot be an owner or co-owner of this property.You can do this by transferring rent to their bank account or pay via cheque. This way you will be able to support your HRA claim properly
2. Usually employers ask for a copy of rent agreement for their records. They can also request for rent receipts to allow you HRA exemption. You can enter into a simple rent agreement with your grand parents. You can prepare rent receipts also.