OK…I uncommented out that line. (and the ancestor of debian-11-dvm is debian-11; i.e., the basic, as distributed debian 11 template, btw).
I then did sudo apt upgrade and it appears to have installed a bunch of stuff.
I shut debian 11 down. I restarted sys-usb. I then assigned the keyboard to windows by mistake and had to re-restart sys-usb to get my keyboard back. I then restarted windows (BTW I did an template/AppVM configuration on that), and assigned the printer to it.
Still nothing. (Unless I bodged the upgrade command because of the template being added? Sorry–I’ve never in my life up to this moment dealt with anything but standard stuff.)