Phil, good to read that you tried... (-: If you could not move the stuck piston using hydraulic force, then I doubt you would ever do so with an air line (were that available) which given the measure of seizure evident could have been dangerous to attempt.
Never known a seized piston to be able to resist real pressure applied to the brake pedal but hey, every day is a school day... (-: Of course you need to ensure that there is no air or flex in the system, and pack the disk to stop it from flexing thus the only component with the possibility of movement is the stuck piston..? Hope you were able to press that pedal like your life depended on it just as tends to be the case in an emergency stop... but as you have been working on the calliper perhaps some air is in the system working against you.
I think it is possible to find stainless steel pistons for Porsche callipers, which may be more suitable for use in seldom used cars..?
Pistons do not retract "under their own power" they are batted back marginally by the disc hitting the pad when the pressure on the brake pedal is released... If they retracted.. then you would have to displace a large amount of fluid for the pads to once again make contact with the discs.... Kinda like when you fit new pads you have to pump the pedal a few times to then make the brakes work, after which they operate as expected.
When you park the car, you naturally put on the brakes to stop it, at which time the pads are in contact with the discs, they do not retract but stay in contact if the discs are wet at the time then that adds complication if the car is laid. IF the dust seals are also damaged and the car has been run that way on damp or wet roads, then that adds further complication, add to that if the brake pads are worn down, then more of the machined surface of the piston may be exposed to the elements and you are into a brake overhaul as opposed to fitting replacement pads and a general clean up of expected corrosion on the calliper and pins etc.
By way of increasing understanding.... Once you have a replacement calliper to hand and have determined there is nothing to stop you from removing the problem calliper by way of irremovable fixings but with all still in place, if you extend the piston that does move and push back the dust seal, you will no doubt then see some of the crud that is restricting it`s movement which can be cleaned off, and which if cleaned and a splash of WD 40 applied during the clean up process might then be easier to push back into the cylinder..? If that process is tried a few times the piston may be less reluctant to be parked back in it`s cylinder... Of course the other likely restriction is the outer ring of the calliper that holds the seal in place and which you can not see as it is hidden under the dust cover.... However if you wish you can pop the piston out and view the whole assembly to increase your understanding..?
Of course YOU do not have the training, certification or the proper facilities to be working on your brakes, and you really should employ professionals to work on safety systems that could affect you and other road users.... So best stick to polishing, though do check that the polish is Porsche approved.... :?:
Had I been so restricted I would have learned far far less than I think I know..