Demand Interest against agreement clause

Hello, I purchased a flat in March-2011 and immediately paid my contribution of 20% of total cost. I planned to take the loan for remaining 80% and informed builder about the same (I have the proof of this communication in emails). They sold me the flat on 15th floor, even though they did not have the Commencement Certificate (CC) for 15th floor. They started the construction as they received CC for lower floors and sent me the demand letters for installments (and kept adding the interest at 24% on delays). I responded to these demands saying bank can not disburse the payment until builder has the CC for 15th floor (these communications are available with me in emails). Builder received the CC for 15th floor in April-2013. Immediately after the CC was received, bank disbursed the payment for all the demands raised until then. I requested builder to remove the interest amount from demand letters mentioning the reason for delay was because bank can not disburse until CC was available. Upon my request, builder removed the interest amount from demand letters sent after April-2013. All demand letters subsequent to April-2013, mentioned "interest till date" as zero. (all the communications were via email and available with me). However, it is time for possession now and in the last demand letter before possession, builder has added huge interest (10.5 lakhs) mentioning the delays on installments demanded while CC was not available (between March-2011 and April-2013). I looked at the registered agreement, and as per the payment schedule in agreement, I was not supposed to pay until registration date (except for initial 20%). Following is how payment schedule looks like in registered agreement: Purchaser to make the payments as under: (1) Rs. 100,000 (10% as earnest money Dt. 31.03.11) (2) Rs. 90,000 (10% on or before execution of Agreement / Dt. 08.04.11) (3) Rs. 500,000 (5% on starting the work of Basement Slab / Dt. 17.05.11) (4) Rs. 500,000 (5% on starting the work of constructing plinth / Dt. 24.06.11) (5) Rs. 80,000 (3% on or before casting of 1st slab of the podium / Dt. 30.07.11) (6) Rs. - (3% on or before casting of 2nd slab of the podium / Dt. -) (7) Rs. - (2% on or before casting of 1st slab / Dt. -) Line numbers (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) --- similar to line (7) for casting of 2nd Slab to 13th Slab --- (20) Rs. 5,000,000 (2% on or before casting of 14th slab / Dt. immediate after registration) (21) (22) (23) subsequent milestones and amounts Basically, for line numbers 1 to 5, amounts and actual payment dates are mentioned. For line numbers 6 to 19, no amounts or dates are mentioned. It just has dashes for Rs. and Dt. Line number 20 mentions 50 lakhs (which is total of installments for slabs from line number 6 to 19) and date as "immediate after registration". But, now they are asking me to pay the interest, for milestone demands sent against line number 6 to 19 during which CC was not available and it was not even mentioned to be paid in the agreement. Builders argument is "if builder had the CC on time, you would have paid interest to the bank. So, you should pay that saved interest to the builder" (I have this in email from builder). They are also saying if you try to go through legal route, it will take lot of time to resolve the case, so I should rather negotiate with them and settle with lower amount. Builder's legal team is suggesting to read registered agreement along with booking memo. Booking memo mentions the payment schedule as per the construction milestones. However, my understanding is booking memo is not legally enforceable and we should go by the terms and conditions in registered agreement Please advise me on the following: 1) Am I legally liable to pay this interest demanded by builder? I have made all the payments as per the registered agreement 2) Could the builder legally sell the flat to me on 15th floor, even though they did not have the CC for that floor? 3) If I go through legal route, how long will it take to settle the case? 4) Builder raised this demand for interest 2 months ago, and we are going through several arguments and negotiations. This is extremely stressful for me, which is affecting my work and health. Can I sue builder for harassment and also sue for loss of rent/delay in possession 5) Should I pay the negotiated amount to builder "under protest" and then send the legal notice for refund? 6) Any other advise / suggestions on how to handle this Thanks in advance for guidance.