On what grounds are you filing for divorce? You haven’t given any reasons for the breakdown of marriage.
As far as the flat is concerned, who has present possession. Please remember, in property matters possession is 90% of the law. Keep record of all payments made against the loan.
As far as demand of Rs.20 Lakhs is concerned, it seems to be just a opening gambit. Your wife does not seem to be too keen on a divorce. I hope you have concrete evidence that she has been living separately for the last two years. That way you can file for divorce on the basis of desertion.