I came across a resource that I think you would find helpful.
https://www.oecd.org/site/adboecdanti-corruptioninitiative/46816797.pdf
http://www.entrytest.com/legal-questions/56/stolen-property-and-punishment-receiving-stolen-property.aspx
According to the Pakistan Penal Code 413, dishonest retention of the vehicle even when a person knows that a property is stolen is punishable by a prison sentence of up to 3 years and fine. The Mardan lawyer should be imprisoned for retaining possession of the stolen vehicle.
I definitely recommend that you hire another experienced lawyer for defending appeals and counter appeals. The case is strongly in your favor.
The fact that you filed the FIR before the sale of the vehicle is another point in your favor. One PW member had mistakenly bought a stolen vehicle in Sargodha. But the FIR was filed in Lahore before the sale of the vehicle, and the car was confiscated by the police even though he had the original files.
But you need to play the cards right. Make a thorough research before hiring a new lawyer and use your judgement.
I have read somewhere that there is a law and then there is the system. You need to play by the system rather than the law in Pakistan. An experienced lawyer will know the best way to play with the system and help you recover the vehicle. He will advise you whether to go file the case in the High Court to recover your vehicle and get the lawyer imprisoned.
I also suggest that you should avoid any negotiation with the lawyer or have any soft sport for the criminals such as the nephew who may have assisted in this case. There should be no mercy for the criminals.