A. In your opinion, you have cancelled POA but, as per the law, POA, can be cancelled by following manner viz.,
1. By way of Issue legal notice by Rigistered Post to the POA
2. By Paper Publication in local news paper
3. If POA registered, you can execute cancellation of POA before the Sub Registrar Office
B. Time is the essence of the contract, hence in the said agreement unequivocally mentioned 3 month period. Hence it will be dormant by itself as per the Indian contract act.
C. Sub Registrar is only stamping duty collector and he is representing the government as a collector of registration fee and stamp duty, hence he can refuse to register, but he has to record the reason on the Book and issue acknowledgement regarding the same. Therefore, the Sub Registrar can't refuse to register without proper reason due to he can do ministerial act not more than i.e. Judicial Officer.