Welcome to the forums. Have you tried loosening the bleeder screw on the caliper? Loosen the bleeder screw and try to push it in with the c-clamp. If it still won't go in, you're probably going to need a new caliper.
The piston being seized in the caliper is probably why the brake pedal is hard to push. Also, the brake pedal may need to be taken off and rust cleaned off the shaft that the brake pedal goes on and greased.
The piston being seized in the caliper is probably why the brake pedal is hard to push. Also, the brake pedal may need to be taken off and rust cleaned off the shaft that the brake pedal goes on and greased.