Leaderboard

As usual Manic is correct. You will need a good breaker bar. Use a wire brush to clean the threads up and Hit with penetrating oil first. Your hub looks corrosion free so your lucky there.

Without the return spring there wouldn't be enough inertia to overcome the cable friction & release the park lever.

The spring is to assist in releasing the park brake. Not sure what the implications of not having it would be. Perhaps it would hinder normal operation. I would put it back and go from there, make sure the sliders move freely.

The service brake applies the piston hydraulically. The park brake applies the piston mechanically using a screw-thread mechanism. If the park brake mechanism isn't assembled correctly, the piston can be forced outward by the park brake mechanism even without the cable connected. You're going to have to retract the piston, assemble the caliper correctly, verify that the rotor and bearings are still good, perhaps replace your damaged pads (NEVER apply grease to the pads!) and then re-adjust the park brake once the whole works is assembled. On my '92 and '93 rear calipers, I used to retract the pistons by pulling the nut and washer off the rear of the caliper, removing the bracket, and then squeeze the piston back in--the screw would just spin in the caliper. I don't remember any particular problems getting the bracket to re-attach with the nut and washer. Maybe those calipers are different from what you have. I haven't looked at those calipers in years.