1. There is no problem to buy the proeprty whose registered sale deed is executed by the GPA, provided the principal is alive as on the date of the execution of this sale deed.
The landlady may be asked to be present before the registrar for sinning the papers along with the GPA holder if you are so much worried about the genuineness of the property.
3. Registered GPA can be cancelled anytime, hence you may get an extract of the encumbrance certificate from the registrar's office to confirm that whether the GPA is still effective ior has been cancelled.
4. You can
5. It cannot be predicted, you may use your wisdom and find out if there any other person holding a GPA, this will be reflected in the EC.
Better obtain a legal opinion from a local lawyer before purchasing the property.