I can't imagine any scenario in which you have FIDO keys and admin enrollment and security but I'm prepared to be enlightened.
a) Because the password is assigned first, it has higher priority, so subsequent logins will prompt for password first until the admin manually changes the user's credential ordering to put the WebAuthn (passwordless) token higher. The user's credential priority overrides the order of challenges in the login flow.
b) There is no option to add or replace one of these credentials, or manage credential ordering yourself, in the end-user webapp that does profile editing / password updates.
An admin may be able to reset your account so that you get the first-login experience again and can enroll new credentials.
Nobody should have designed something like this, for WebAuthn in particular the standard is explicit about the desire for multiple tokens. Lots of the design is more complicated so as to support that capability.