Well, I worked with it a little more and now I've changed my mind about the cause. I don't think it's the Firing Pin Lock Spring any more. Either the Hammer Spring is much stronger than I realized or there is another issue in the linkage. Perhaps the Hammer Strut got misaligned somehow when I reinstalled it (but I really doubt that because the DA/SA both still function fine, just really heavy) or some bit of detritus got into the linkage as I was working on it.
I still want to replace the Firing Pin Lock Spring, of course, but the Hammer Spring is taking a higher priority than I'd thought. And I may have to bite the bullet, take my fate in my own hands, cast fortune to the winds, and man-up & pull out the entire trigger system for a cleaning and light polish. :P
I haven't even shot the thing yet. I just want a DA that I don't have to use both fingers for and a SA that feels less than 10 lbs.
Peace favor your sword,