Possession letter and maintenance charges

I have purchased a flat admeasuring 394 sq feet carpet. and executed agreement in Sep 2014 under Maharashtra Ownership of Flats Act 1963. As per agreement possession was to be given by Mar 2015. Builder issued me a letter in Nov 2016 enclosing therewith a copy of completion certificate dated Apr 2016 issued by Municipal authorities and asking me to pay last instalment of the agreed price. The said letter was actually received by me by hand only during 2nd week of Dec 2016. I had paid full and final sum on 24 Dec 2016. As per the terms of agreement, I am supposed to pay one year maintenance charges/outgoings at the time of taking possession @ Rs.2/- per sq feet which works out to Rs.788/- per month. However, builder had insisted upon me to pay @ Rs.1400/- per month and for 18 months instead of one year and that too from June 2016. I paid the same on 02 Feb 2017 whereafter builder had obtained my signature on a non-judicial stamp paper addressed to the builder in connection with possession of the flat wherein there was a provision for signature of the builder that possession is handed over. However, builder have neither signed the said paper nor issued me the letter of possession though keys of the flat have been handed over without executing any paper formalities. Despite repeated requests, builder have not yet issued me POSSESSION LETTER/CERTIFICATE. Further, as per the terms of the agreement, housing society would be formed and if the same is not possible then condominium will be formed. Now the builder is insisting that I must execute DEED OF APARTMENT for formation of Apartment Owners Association under Maharashtra Apartments Ownership Act 1970. It is more than 1.5 half years since I have paid full consideration of the flat. My querries are as follows:- (a) Whether builder can charge for maintenance in excess of the sum agreed upon in the registered agreement ? (b) When agreement says that I shall pay one year maintenance in advance at the time of taking over possession, can he collect the same for 18 months ? (c) When builder had issued me letter in Nov 2016 asking to pay last instalment, can he charge maintenance from June 2016 ? (d) When builder have not yet issued me POSSESSION LETTER/CERTIFICATE, can he charge maintenance ? (e) Whether provisions of RERA are applicable for the building the construction of which was completed before 01 May 2017 ? (f) Whether builder can submit the flat/building under Maharashtra Apartment Ownership Act 1970 when the agreement for sale was executed under the provisions of Maharashtra Ownership of flats Act 1963 ? What remedies I have against the builder for the unfair trade practices and deficiency in service. MY email id is - [deleted]. Kindly send reply on the said email id.