Inspection of flat denied even though 95% paid. Buider demanding to pay the remaining full amount

We have booked two flats in 2018 in Navi Mumbai. it was supposed to complete by dec 2020. As per the payment terms 95% need to be completed by the completion of slabs and the last 5% at the time of possession. 95% paid for 1 flat by bank and 87% paid by us for the second flat. There was a agreement with the builder for the second flat to pay the remaining 8% at the time of possession along with the 5%. The persone who was dealing with us assured us that they will not charge interest for this 8%. Now they send the IOP with the interest calculated for both the flat even for 1 day delay (They set the invoice due date for only two weeks), along with all the other charges with exorbitant amounts.( like Maintenance charges like 10000 rupees per month for two years, even though in the agreement it was only 6000), stating that we should complete all the payments immediately, only then they will arrange the inspection within 15 days after the payment, however we should take possession within 30 days from the date of invoice, if not we have to pay them for retaining the flat. Now if we call them, they will not pick up, if we sent mail, will not get any proper reply. We are not residing in Maharashtra and cannot travel to Mumbai at this point. When we asked them to send us the copy of the Occupancy Certificate, they are saying if we go to their office after taking an appointment, they will show us, knowing well that we cannot travel. My question is, is it legal for them to deny the inspection of flat before the final payment? Is OC is a confidential document that they cannot send the scanned copy through email? Should we have to pay the interest for such minor delays? They never send the interest component in any of their previous invoices, only in the final invoice they accumulated all the minor delays and included in the final invoice.