Note that this CSS change could cause some abuse problems. If usernames are hidden, I could change my full name and avatar to match someone else’s, and without a username it becomes much harder to see the difference.
when I mention someone, I can search by usernames and real names, but Discourse makes me see just real names, perfect
when I choose the user to mention, in the editor/forum will appear the username (and it’ll be clickable): is possible to make the real name visible in these cases?
If you disable the “enable names” feature, they won’t have names, just usernames. You’d have to settle for _ rather than space.
Or you could write a custom plugin to do what you think you want.
If you don’t disable them as I suggest, you could hide the name changing stuff with CSS and have a community rule that if they changed their name (by un-hiding the CSS in their browser) that they would be appropriately punished.