Have you tried adding GRUB_TERMINAL_INPUT="usb_keyboard" and GRUB_PRELOAD_MODULES="usb usb_keyboard ohci uhci ehci" (see note below) in /etc/default/grub and then the command 'grub-mkconfig -o /boot/grub/grub.cfg' to regenerate grub.cfg? That should add various additional 'insmod' lines in grub.cfg, for those GRUB modules.

Note: Those GRUB modules are in the directory /boot/grub/x86_64-efi/ if your PC has UEFI, or /boot/grub/i386-pc/ if your PC has PC BIOS.