As a last ditch effort I would use a channel lock to pinch off the brake line, disconnect it from the caliper and remove any park brake cables, and with the caliper now free of the car, put it in a vise and try to turn the piston back in, there would be no pressure to resist it going into place, even if the piston came out too far you should be able to move it in. If no luck you should be able to take the caliper to brake shop and avoid having to tow the car. I have had this happen on one of my Mustangs (very simular caliper) and just having the caliper off the car made it easy to work on. When you reinstall the caliper and brake line then remove the channel lock and then just bleed out that caliper. Some times there is a rubber o-ring on the piston and if it comes out too far the the o-ring will come out of the groove and keep the piston from going back in, also the seal/boot can come out of its groove and be the cause for the piston not wanting to reset. Good luck and just go slow and it will work out, or in as it may do.