Thank you for the ideas, bcworkz.
Thinking about your advice about the hook order, I consolidated all of my register functions into one, which registers everything in the appropriate order, to eliminate that as a problem.
It still didn't work, so I figured that I would simply call the register function at the beginning of my key display function, where the taxonomies failed to exist. And this worked, insofar as the taxonomies now exist at that point.
However, it killed my WP_Query, and I can now not query even the questions. have_posts() is returning false;