Registration of FIR by police on a 29 years old mutation/sale deed

Respected Sir I am in a very peculiar and baffling situation. My grandfather bought a piece of land from Mr P in 1989 by following due course of sale deed and property registration. The mutation was successfully sanctioned on my grandfathers name. In 2015, Mr P registered an FIR (collusion) with the Economics offence wing of Delhi Police stating that sale deed is forged and he has not signed/thumbed any document. Police started issuing us notices asking to be part of the investigation. We have provided the IO with all certified copies. Now the issues 1) Since couple of months, Mr P along with his entire family has been sitting on that land refusing to budge. He has started encroachment and has recently raised a wall boundary (land was without boundary, just wired fencing) 2) My grandfather had been serious since 3-4 months. And with succession issues we could not register a complaint. Finally he has registered the GPA in my name. 3) Mr P's FIR has become a pain as he also colluded with revenue officials (patwari), and was able to get an objection raised against the property, stating there is an FIR against the owner. Thus I can not get a No objection certificate for sale deed. What should be done legally in terms of 1) Getting the FIR quashed 2) whether any writ petition such as mandamus can be of any help 3) action against revenue officials for admitting such an objection 4) how to proceed criminal charges of encroachment, threat against MR P, and evacuating him off All help will be much appreciated