Delay in Possession of Flat

Sir, We made an agreement for buying a flat in Sept 2009. The agreement says, the flat will be handed over by end of May 2010. All the payments for the purchase are already made, well in time. Now, after more than five years (May 2010 to March 2016), builder is waiting for 'Occupation Certificate (OC)' from Municipal Authorities. When we asked the builder, for handing over the flat with 'OC', and conveyance of land part to us, they gave keys of the flat and a letter, saying, 'Keys of the flat are handed over, to make furniture only', and 'not allowed to make any structural changes'. There is no mention about handing over the flat and conveyance. Our questions are, 1. With such letter, for making furniture, can we stay in the flat / rent the flat ? 2. Without 'OC' from Municipal Authorities, is it legal and correct to stay in such flat ? 3. Can we ask for compensation from Builder for delay in possession of the flat ? 4. Most of the flat buyers are residing in this building, without 'OC'. I was told, this is the practice in Mumbai. Is it OK or not ? Thanks in advance.