Building without OC , CHS Society formed

Dear Sir, I am from Navi Mumbai, puchased flat in 2014 from reputed builder. In Apr'2014 I did flat deal with builder, per agreement I paid him via HomeLoan. In agreement, he promised OC(Occupancy Certificate) in 3-4months. Builder had given Possession letter mentioning "Possession for interior work". Till date Jan'2020 OC has not came, builder is giving excuses of delay due to NMMC. Per RTI in NMMC, builder has not done work per approved pan few points not complied. We(all flat members) have formed CHS society & registered it. We are operating building by collecting maintenance charges from each member & collecting property tax per sqft calculation. Property tax bill is comming on Building name(not on individual name ). Requesting your help for these 2 questions. Q1) Due to not having OC , banks are not giving loan to new buyers, so resale value has gone down. What we should do to Obtain OC? Fyi- expensive lawyers are not preferred by members, as per other friends similar experience they are more interested in dragging case many yrs. Though pls advise per yr positive experience. Q2) Builder has kept 2 flats on his name, though sold but not registered on buyer name. Now this flat owner is reselling the flat. On paper, the flat is on Builders name & deal happen between new buyer and Builder . Can society charge Transfer fees for this flat transfer? Thank you.