And for posterity: that worked just fine.
# efibootmgr -c -d /dev/sda -p 1 -l \\EFI\\opensuse\\grubx64.efi -L opensuse
as root wrote the opensuse entry back in the UEFI boot entry table and now it boots properly again. Phew. Sorry to be so episodic about the whole thing. I figure my learning struggles may help someone else at some point. Thanks again nrickert!