A positive but confusing update: after spending about 8 hours installing, activating, testing, deactivating and uninstalling several dozen other WordPress gallery plugins, I tried reactivating WP Photo Album Plus (I never uninstalled it, just in case) and suddenly it is working.
I have no idea what may have fixed the problem, but now that I've been able to test it I can confirm that it is exactly what I am after. Thank you for doing such a great job with this plugin.