With r30659 the first part is fixed (Nosuchusershort is used when account creation is not restricted). The second part is not applied, because MediaWiki:Login is "not" to be used as a header (while MediaWiki:Userlogin is). MediaWiki:Login is used as the caption of the grey box in which the username and password fields exist. It may later be decided to change its text, and that should not affect the behavour of the page header. Only because its text is what you desired doesn't mean we can use it as a header.

I'm not introducing a new message for that either. The reason is, if a wiki admin chooses to disable account creation, he can also updated MediaWiki:Userlogin on his wiki and remove the "/ Create account" part. That is why we have messages: to let admins easily update them.