I think I have tracked down the problem. The drive shaft gear has a slightly smaller space between the rings of teeth, and this is binding on the fork in some positions. I have reinstalled the original plastic gear, and it appears room be much smoother, at least when turned by hand.
Now that I have a decent measuring setup, I can see how little it takes to go from "not tight enough" to "too tight". I need a better second wrench and a way to lock the spindle in place, and then (maybe) I can get this part of the project wrapped up.
For the record, I made marks on both the spindle and the drive shaft with a sharpie when I felt drag. After a few rotations, it became obvious that the problem was on the drive shaft. Once I knew where to look, I found I could see the fork shifting here and there. Manipulating it by hand while rotating the shaft made things a bit smoother. Rather than trying to grind and fix, and maybe ruining one or both parts, I decided to go back to the plastic gear and save the repairs until I gain a bit of experience (or wreck the plastic gear and amore forced to make the swap...).