Removal of confusion regarding Bhara Salami

Hello Sir, My grandfather R R Singh rented a flat under bhara salami on 16/03/1986 in Kolkata from D P Das (Original landlord). As salami he gave an amount of Rs. 40,000/-. The rent was fixed at Rs. 400/- per month. The rent receipt was issued in my grandfather's name. After a few years the original landlord i.e. D P Das passed away. The property was inherited by his children i.e. C Das & S Das. My grandfather passed away on 09/03/2018. After my grandfather's demise, the now landlord C Das & S Das refused to issue a rent receipt or change the name on the rent receipt & asked us to evict the flat as the original tenant i.e. my grandfather has passed away. After going through a lot of trouble, somehow we have managed to give our rent to the Rent Control. Now, I would like to know in detail:- 1. What is bhara salami? 2. What is the law regarding bhara salami after the demise of the original tenant? 3. Has my father inherited the said flat legally after my grandfather's demise or do we need to evict the flat after 5 years from the date of death of my grandfather? What is the law in this situation? 4. The landlord is very cunning & wicked. Under no circumstances he is willing to change the bill in my father's name. How can we retain the position in the flat legally, even after 5 years? 5. Can we sell the said flat to somebody else, even without the approval of the landlord? 6. My grandfather has lost the original deed. Now we only have the original rent receipts. Is there any way to get hold of the original deed? Other suggestions regarding the problem in our situation is appreciated. Thanking You, P Singh