Unjustified interest charges, deficiency of the service, unfair business practice, harassment,

When I booked the under construction flat, initially lot of miscommunication happened for that they charged me interest for the amount due and not paid in time but all that was waived on Oct 20,2020 with the condition that rest of balance payment which was 30% to be paid in the time. The agreement was that Quote ."we ( Piramal) will ensure that the demand letters are been sent out to you (me) accordingly. " Unquote Builder was sending the demand note only by email which I objected initially but they agreed that once the demand note is sent by email, they will inform me by Phone call/SMS which they did also for demand notes but for one demand note (95% of CLP, dated Dec 27,2022) neither they informed me nor that was told me in any of my discussion /meeting/ phone call after it was raised. There was no follow-up/reminder/ message/email even when I visited the project site on March 27th, 2023. So from Dec27 2022 till April 11, 2023 there was no discussion for the delay. Final (possession demand letter) was issued to me on March 30, 23 and it came by courier. On April 11,2023,during our virtual meeting ,I was informed was a Demand note generated on Dec 27 2022 which I did not pay in time hence, conditional waiver conditions not met and I have to full interest amount. When I raised it then they asked me to pay the balance amount minus the interest amount and they will revert on the interest part. Once they received full due amount as per Possession demand letter, and I asked for the possession they informed that full interest amount to be paid and it will not be waived as condition of waiver not being met. I didn't receive the demand note and as per agreement they did not inform me. They just send by the email. All payment baring one, I made all the payment in time. Dragging the last payment then demanding the interest amount. Holding my possession of the flat till now. My question: Will it not come under deficiency of the service from the builder, unfair business practice, deliberating delaying the possession. Can I file case both in RERA And Consumer Court and demand for the compensation after paying the interest under protest and take the possession of the flat. Please share your number