Regardless, I can now hand-load all 13 rounds into the mag, and all 13 rounds will pop up when I "thumb" them out. They don't all pop up with the same force, so I still have a little more to do, but I think this is going to work.
Just a final update. I went to the range a few days ago, and with both factory mags loaded fully, the pistol fired fine, with no failures of any sort.
When thumbing rounds out of the magazine, the rounds still don't all "pop up" with the same amount of force, so the follower is still dragging a little bit, but I think the recoil when firing is allowing them to feed without the follower sticking enough to cause a problem.
I should point out that it's entirely possible that fully-loaded magazines would have worked perfectly reliably from the very beginning, even without the work I did to smooth the contact points between the follower and the magazine. But considering I couldn't physically load 13 rounds into either magazine before smoothing them out, I'll never know.