Can i fill this gun with a regular air comptesser with the right adapters
No, a standard air compressor does not put out the necessary pressure to fill this gun.
Is the reservoir detachable?
The P1 Manual shows an open sight but the specs say there is no sight included. If the pistol does not come with an open sight, can I purchase the correct one(s)?
A standard (non-QE) version is available, and that gun has open sights installed.
You would need to contact Hatsan USA regarding sights
Would my air hand pump for my Benjamin Maurader work with this pistol without an adapter?
No, you would need Air Venturi Male Quick-Disconnect, 1/8"... to go on the end of the fill probe that comes with the Hatsan to make it work with your existing pump
Does the pump come with it or u have to buy sepret
You would have to purchase the fill source separately.
Is the magazine and air chamber same as bt 65 ?
Air chamber NO Mag YES