Mandar was officially declared dead. So today he is a dead man in eyes of law. First Mandar will have to file a case in Court to get himself declared as alive and to seek declaration of death certificate as null and void. Here Pallavi was a widow in eyes of law at the time of her marriage and hence her second marriage is legal. Suppose if there was some property which belonged to Mandar upon his death it devolved upon Aai and Pallavi as per and they both sold it. The sale is legal. Similarly since Pallavi has acted upon in view of the legal death certificate her marriage to Raghav is legal and valid. Firstly Mandar will have to go to Court to prove he is alive 🤣. So legally RaghVi marriage is valid. Any lawyer advising otherwise needs to relearn law