Turns out, my little horn pin/spring/collar assembly was installed incorrectly. I had left it the way I found it when I took the wheel off. Turns out, someone along the line had put it back together wrong, and I just didn't know any better. It was in the order of spring, collar, pin, with the large side of the pin facing out. It should have been spring, pin, collar, with the large side of the pin facing down and the narrow side extending out of the cancel cam tube about 1/2" or so. At least, that's what I was told elsewhere today.
I want to get that black plastic cover off of the column so I can get to the retaining ring and plate, but prying from the slots just seems to bend things. The thread on here that shows how to take one of these columns apart says to pry from those slots, though.