Should 'seller" be made a confirming party in sale deed?

I had bought a flat from Party A by signing an Agreement to Sale with Party A. Since the Sale Deed for the Flat was not signed between Party A and the Builder, the Builder was made the CONFIRMING PARTY in the Agreement to Sale. Now, I have to sign the Sale Deed with the Builder to convey the title of the said Flat to me. One of the clauses in the Agreement to Sale says that "The Final Deed of Sale if required conveying the SAID PREMISES along with the proportionate share in the land shall be made by the CONFIRMING PARTY directly in favor to the PURCHASER, and it shall be at the discretion of the PURCHASER to join the VENDOR as Confirming Party in such Deed of Sale. In the event the PURCHASER desires to join the VENDOR as Confirming Party in such Deed of Sale, the PURCHASER shall take necessary steps to procure the signatures of the VENDOR in such a Deed." The Builder (CONFIRMING PARTY in Agreement to Sale) is insisting that we must include the Original Vendor (Party A) in the Sale Deed as CONFIRMING PARTY. However, Party A has expired and her legal heirs refuse to co-operate in signing as CONFIRMING PARTY in the Sale Deed. I need advise of learned advocates on this forum: 1. Can I sign the Sale Deed with the Builder without having the Original Vendor (Party A) as CONFIRMING PARTY? Can the Builder insist on having the Original Vendor (Party A) as CONFIRMING PARTY? 2. Is it required to file a suit for specific performance against Party A and get a court order before proceeding to sign the Sale Deed without Party A as CONFIRMING PARTY?