1. Yes the stay order need to be challanged in high court as it is interim order you need to file a civil writ for vacating the stay and restoring status quo. The high court shall decide the case based on merits and documents available.
Since the only thing is if your grandfather died without will then the property shall be divided in all legal heirs not only on grandmother so you need to claim it was oral will bt grandfather to give property to grandmother and then she executed will in your fathers favour.
2) that can be decide on just a view that how the property from grandfather was transferred to only grand mother if there was no will oral or written what is your stand in court???
If that transfer is valid than gift is valid and you will win the case as other relevant factors like possession statements are in your favor.