Alright, well, I managed to accidentally soft-brick my machine by overwriting both the active and backup GRUB configuration files. Fixed now.

While trying to fix it I encountered such absurdities as the keyboard only working in the BIOS on one USB port but only working in Linux on another, the UEFI shell apparently lacking a pager, and Linux being unable to display anything unless GRUB had already switched from text to graphics mode.

Eventually I refined my series of incantations and keyboard plug relocations to the point that I could boot Linux and copy a new GRUB configuration into place. For safety I now have a second GRUB backup in the EFI partition.