1. you are the legal heir of the original deceased tenant. so you are entitled. no new agreement is required. the legal heirs of the deceased tenant become the joint tenants by operation of law. It does not matter that the rent receipt is still issued in the name of your grandmother who was the original tenant
2. there is no need to pay any money to the landlord's son. however if you want the rent receipt to be issued in your sole name, then you can take noc from other heirs and submit that to the landlord and request him to transfer the rent bill in your name, however for that you dont have to pay full 33% since the heirs get tenancy rights by operation of law and there is no transfer of tenancy to a third party as such. in case you wish to buy the remaining rights of the landlord in the unit then for that he may demand the market value of the unit, since by transferring the remaining rights in the unit, you would become the owner of the unit.
3. what rubbish! just continue to pay the rent to the heir of the deceased landlord and take proper rent receipts from him. If he is not accepting the rent then do a money order in his favour or make an application to the small causes court to deposit the rent in court since the landlord refuses to accept the rent from you. the builder as and when appointed has to recognise the heirs of the deceased tenant and the new rehab unit will be allotted to them only or to any one of them as per the mutual agreement between the heirs