The mainspring screw should be all the way in and tight. It was never designed to be adjustable. Post #8 (photo of the hammer) shows that someone has messed with it. The trigger sear should rest on a slight shelf that keeps the trigger slightly off the cylinder stop locking the cylinder with the hammer down. In this photo, the shelf has been broken/filed off. This shelf acts as described, allows for the rebounding feature and also functions as a half-cock notch. A strong pull on the trigger when in the half-cock notch will break this shelf and allow for the freewheeling you describe. The red arrow points to the location of this shelf. Also in your photo; the full-cock notch looks OK but worn. The test is to slowly pull the hammer back to the full-cock position. The cylinder should lock an instant before the hammer catches. The trigger sear profile appears good. My opinion is that you need a replacement hammer. Welding, re-cutting and hardening the hammer is an option but winning the Lottery is easier.