...dammit.
Yeah, OK. I went in and set Posts Per Page to 2. In theory, this should create 6 pages based on the number of future posts, or 3 pages based on the number of published posts. But it doesn't - it creates 2 pages, and then the third one 404s. In fact, this happens regardless of how many PPP I have - even if I set it to 1, the number of pages ends at 2.
I'm starting to feel that my best option is going to be to rework the structure of the site so it sorts published posts by a custom field that displays the future event date - from my reading it looks like that would be an option, although not an ideal one. I'm kind of going crazy, because I can't tell what exactly is broken that needs to be fixed.