You are correct in that I failed to note that the gun is a three screw. But you confused things by referring to the hammer pivot pin, which does not exist in the three-screw guns, which have a hammer screw. A slave pin cannot be used for the reason you state.
In the three screw, to replace the hammer screw, you only need to remove the grips, take the tension off the hammer spring in the normal way, then remove and replace the screw. If you don't move anything, the new screw should go right in.
Jim