Delay in completion of housing project and overcharging by builder

Hi I had booked a unit under a housing project which was to finish in March 2016.I have paid all my dues timely but the project was delayed.In the last instalment at which occupation certificate was to be given ( Oct 2019) , the builder asked for for cost escalation, development charges, maintenance charges, increased area ( without increasing the carpet area). I want to ask the company to pay me interest on the delayed period and drop the unnecessary charges imposed under various heads mentioned above. What is the best way to secure that. I have read about cases where RERA has directed the builder to pay the amount to the buyer but , builder has defaulted and not paid any money as RERA has no financial restraining power on builders. 2. in the Meanwhile my mother who was co allottee died and the company took almost 1 year to delete her name despite my providing them documents as demanded by them. That led to my inability to pay the last instalment in time because it was not possible to get loan from bank in the name of person who was dead but existed still as co allottee . Now , finally the name is deleted. The builder has added an interest at rate of 18percent p.a to me over and above the last instalment amount due to this delay . Does this non payment of last instalment ,goes against me if I take the case to RERA. 3. Is it true that NCDRC entertains cases above 1crore and NCLT now only admits cases if we go as group of 100 or 10 percent occupancy of the residential project. 4. What are other options that I have to secure my principal amount alongwith intwrest which does not involve long court cases. Thanks NJ