As per the description, the rescue shell (if you can't boot) will very likely not work if you have a USB keyboard. Expected result: you can type "shell" and troubleshoot if you can't mount the rootfs.

I believe this is due to not having "usbcore usb-common" in the USB modules list, and debian-sources currently builds these as modules, so they would need to be included on the initramfs or alternatively enabled in-kernel.