Can you post "sudo ls -alR /etc/letsencrypt/accounts/"? (The contents of your private key files are secret, but the list of files isn't sensitive.)
That error is happening because you downgraded to a much older version of Certbot.
stretch-backports contains the latest version of Certbot, so I'd suggest enabling it and upgrading.
You can work around that issue without upgrading, but I think upgrading might also help with some of the other issues.