Can't login to Ubuntu Core 16 using a second ssh key
Solution 1:
The SSH keys are imported once during console-conf into your user's home directory ~/.ssh/. Therefore, if you mount the writable partition on a different Linux machine you can manually append the missing SSH public key(s) to /writable/user-data/username/.ssh/authorized_keys.
This worked on a WDLabs Nextcloud Box running Ubuntu Core 16 and Nextcloud 11.