So, I guess I don't understand what would be being broken. With the change and a kickstart like I described (username set via kickstart, password not set), the user spoke will focus on the Full Name field. Without the change, it will focus on the password field, which may or may not be correct, and more troubling it would obscure the "The password was set by kickstart" placeholder text.
As far as the second conditional, it is not possible (both in GUI and kickstart) to set the full name without setting the username, so that statement would only be true if both the username and the fullname are set. And in that case I don't understand why it would be helpful to focus one field over the other. Why not just have the spoke behave as it always would?
And the advanced button sensitivity is handled in the username_changed signal handler so the set_sensitive call there is redundant.