Delayed possession

I have a flat booked with Amrapali in Noida. Booked in 2010 July, the agreement mentions the delivery commitment as " 36 months +- 6 months, from the date of start of excavation / signing of agreement". I does not mention anything about whichever is earlier or later. The agreement was signed in September 2010 and excavation work for my tower started in Jan 2012. I have not been given the possession yet, since 2013 December I have been getting assurance of possession in next 3 to 4 months. Amrapali has given possession to many people in the society but few towers are not yet delivered. People are staying in other towers since 2014 mid. I was later in 2014 informed that my tower is in phase 2 delivery (but there is no such phase 2 mentioned in agreement). Actually my towers is almost ready for the last 2 years but there is no progress in it since the last 2 years (approx). And my flat has been made ready completely with tiles and interior work done, and is being used by Amrapali for their office purpose (the facility management office for society). I have raised my objection to my flat being used by others and have requested that if my flat is ready I should be given possession. I am as of now paying rent and also paying EMI. Builder is also not paying any late payment charges as of now, and saying it would be settled at the time of possession. Have below queries in this regards: 1. Is my case legally strong enough / genuine enough for claiming the late payment charges from builder. And is the late payment charge settled at the time of possession only, no provision of getting the late charges as and when the payment is delayed? 2. Builder has also charged me on some late payments, I tried to ask for the exact amount of late payment so that I can settle it, but they said it would be settled at the time of possession, I believe they are trying to negotiate during possession for my late payment amount to their project delay. What could / should be my step on this, to avoid falling in any of builders trap. 3. Can I claim additional rent / amount under any other head from builder for using my flat for their office purpose. 4. I have been coordinating with an employee of builder over emails for all these follow-ups, I do not have any of their managements correspondence, is it required to make the case strong. 5. Can I approach the court, if yes, which court, and what do I need to prepare for. 6. Is it meaningful to send a legal notice to builder asking for the damages, and is it fruitful means in this circumstance.