Builder is disputing over 7/12 mutation application

My mother died just before 8 months and my flat was registered under RERA ACT. Now conveyance deed is mentioning both names of my father and mother jointly held. So we went with tehsildar and tehsildar told me to get "pedhinamu" called legal heir certificate. And for getting pedhinamu , you will to mutate first your flat in mamlatdar office ( municipality office). So we went to the municipality office and we just gave them mutation application called 7/12 dakhil arji ( entering name for mutation application). And after 15days that mutation application converted into the dispute application. We asked about this in municipality office and they told us " The builder gave us dispute application. You will get dates on which you need to visit "mamlatdar(municipality court)" for it on proper dates decided by mamlatdar court. Now they gave us first tarikh and that builder was absent. And he told me here is dispute application from builder , copy it and give me back original. And come with lawyer on next visit , if you dont aware about laws. Now in that Arji ( application) , builder wrote that " please remove mutation of the flat no. 101 , xyz residency under this application. Because of xyz residency name is coming in this mutation application." And then we visited to the builder office and he told me that " Sir why you mutated flat now? We sold you flat in 2017 , and in 2019 still some blocks of flates are not sold.So when society is form , we will handover this residency and then mutate your names after 1 year . " So i told them " then sir what about pedhinamu , my mother just died 8 moths before and we want to remove her name and want to add name of legal heirs. So for that we need pedhinamu called legal heir certificate . And for that legal heir certificate , we need to mutate it first and then remove name from legal heir certificate as told by tehsildar. Then builder gave us no replay and just ready for visit other sites and just runaway . Now sir any solution for it?? 1.) So what about my legal heir certificate called pedhinamu in gujarati ?So can we get it after 1 year?? 2.) My mother died and we just financially cracked. And we want to sell flat , so we need to wait for 1 year?? 3.) And what about that dispute application given in mamlatdar court(small court ,municipality court) from builder. And how to give them respond in legal way??