What can be done now

When both parties filed written statements in a court and a case is pending, it is the court that has to decide.  As you have already engaged a family lawyer and proceeded as per his directions from the beginning, seek further guidance from him as he is the only man well versed with documented facts and your late father has done as per his directions.  Though the owner and your father expired, the trusted party by your deceased father is still living and explore avenues for compromise.

Yes, the legal heirs could be substituted as the parties. However, chances for a favourable outcome is too remote because the facts are less than benign and particularly because  of the provisions of Prohibition of Benami Transactions Act.

You have made a mess of facts, re-post in simple language.It is better to consult a local prudent lawyer for appreciation of facts/ documents and professional advise. 

This is a case of jumping from the  frying pan to the fire directly.

Legally you cannot fight against the so called trusted party at this stage.

A compromise would be the solution that may be suggested now.

Your father was a GPA holder, hence upon his death the GPA automatically stands cancelled, in any case the legal heirs of GPA claim any rights in the property since the GPA himself was devoid of title.