Point No.3 is absolutely right. In fact it is not known that how did the principal revoke the POA deed , also you have not mentioned whether the POA deed was executed by a registered deed.
In the event of the POA deed executed in favor of the agent was by a registered deed, then the cancellation cannot be done unilaterally by the principal alone, he may issue a notice to the agent intimating his intention to cancel the same and both have to execute a registered conceptional deed jointly.
Unless this procedure is followed the POA deed cannot be treated as cancelled even though the principal had issued a notice instructing that the POA deed stands cancelled.
In any case the broker's suggestions (point No.5), is not legally valid, because the property was sold by the POA agent during the lifetime of the owner especially when the POA deed given to him stood cancelled, making the sale deed executed subsequently as invalid.
Therefore the option before the seller is to first get the sale deed executed in his favor ratified through the legal heirs of the deceased owner by executing a registered ratification deed, provided the local sub registrar accepts the same at this belated stage.