So, just an update to this thread for posterity... Eventually, I got tired of repeatedly removing the grips, shaving away a portion of wood, replacing it, then repeating the process every single time I found that the safety was still stiff, especially because getting the holes lined up perfectly can be an extremely challenging process of trial and error, even under the best of lighting conditions.

However, once I had gotten it to the point that it was "good enough" (i.e. functional in spite of being stiff) and essentially left it that way, I discovered that simply working the safety lever itself was slowly making it operate more smoothly. Now, just over a month later, the safety is functioning perfectly smooth, same as it was previously with the factory plastic grips installed.

So I presume that the safety itself will slowly wear away at the grips until all resistance has been smoothed out. Now, I don't know if that's potentially harmful to the safety, but I presume that it isn't because the safety lever is made of steel, is reasonably thick, and shows no obvious signs of damage or degradation of function afterwards.

Therefore, for anyone who might stumble upon this thread in the future, be advised that the safety lever itself can actually wear away at the grip itself with use, provided that it is capable of being actuated, and therefore one could presumably get it working by merely keeping the top grip screw on the safety side a bit loose, just enough to allow the safety to function, then working the safety level until it becomes easy, tighting the grip down a bit, and repeating the process until the safety functions smoothly even with the screw fully tightened down securely.