So the exact problem (if memory serves) was that the function output HTML to be consistent with the Twenty-Eleven theme, and I’m using a custom theme, which uses different HTML tags (or at least differently arranged HTML).
Thanks for the tip on renaming the function and shortcode in rsvpmaker-custom.php, so I don’t have to disable the original. Once I renamed the shortcode called in the site’s Upcoming page, that solution seems to be working for me.
