Yes, the configuration of the domain mailcard will be valid for all users who have not configured their own personal mailcard.
No, I have described how to get to the domain mailcard configuration by going through the admin panel … you can use any user to configure it and it will be valid for everyone 
Alternatively, as has already been explained to you, you can add specific authorization on a user and use that for the configuration of the domain mailcard.
I hope everything is clearer now 
BR