Eventually sorted out that
$ xdg-mime default org.gnome.Evince.desktop application/pdf
fixes the problem, per
https://wiki.archlinux.org/title/GNO...ocument_viewer . This appears to modify only $HOME/.config/mimeapps.list in my setup. And now calibre is opening PDFs in evince once again. A bit annoying that I will never know what random package I installed may have decided to replace this association, without asking.