you might need to temperately switch to user/password login and then switch to ssh keys

follow this
http://support.hostgator.com/articles/specialized-help/technical/how-to-disable-password-authentication-for-ssh
but set it to yes instead of no

You can't.

You can't provide SSH key to do DO Console. First of reason is that Console is not an SSH connection, it doesn't look at sshd_config. It would be like that you attach screen and keyboard physically to the server.

Now if you followed initial server guide, you probably have non-root user, e.g. sammy. You can use that user and password for it and you'll be able to login without hassle.
Problem is if you don't have non-root user and don't have root password. That would require you to use Reset root password. That option will email you new root password which you can use for login.
Make sure that when you login with it, you'll be asked to change it. First you must type old password, then two times new password.

I think this is better solution than recovery ISO as you don't have to wait support for mount/unmount, down time is much less and you work directly on server.
If you get any problem with it I will try to help you