1) you can file complaint before consumer forum against builder for unfair trade practice for demand of Rs 2.61lakhs against alleged increase in super built up area . you can also obtain orders that builder be directed to handover possession of flat and obtain OC from muncipal corporation
2) as per your allotment letter area of flat is 1851 square feet and you have paid 95%of flat price . at most you are liable to pay balance 5%only . demand made by builder for increase in contribution is unjustified . please note builder cans ell flats only on basis of carpet area .
3) in your case sanctioned plan has not changed to justify demand for increase in payment by flat owners . claim for interest at 24%is unjustified
4) builder can give possession of flat only after obtaining OC
5) you can in alternative pay money demanded under protest take possession of flat them move consumer forum for refund of money paid with interest