Hi,
If you’ve lost the private part for your public SSH key on Launchpad, then you will not be able to log into an Ubuntu Core device using SSH.
From here I would recommend removing the existing public key (it’s useless without the private part), and generating, then adding a new SSH key to your Launchpad account. Then, when you sign in on a device, that public key will be imported into the device, and since you now have the corresponding private part, you will be able to log in.
I don’t think you need to rebuild the image, since as you mentioned, the SSO account sign-in is done on first boot, not on image build, but you probably need to reimage the device (you can use the same image you built), so you get a chance to do the “first boot” again.
- Daniel