For future reference; kludged together a workaround when microsoft support couldn't help, by using the fact I have two disk drives.
Took out my SSD, installed windows with a new account on my old storage drive (RIP to all the games on there), plugged SSD back in, dragged everything in my original User folder over to the new one in a zip file, unplugged HDD, reinstalled windows on SSD and was able to set a local password, plugged HDD back in, got the files back over. It wasn't perfect, and there's almost certainly stuff I missed, but it worked.