I'm able to get any domain working without the plugin as well.
- I've got a subdomain multisite installation.
- I add a new site, giving it a dummy subdomain.
- In the notice, I click on "Edit Site".
- I change the domain to whatever.
- If I visit the site immediately, I get a fatal error. But it's in a plugin (google analytics).
- If I go to /wp-login.php, and then back to the homepage (without logging in), the error goes away.
- I can log in fine.
The domain mapping plugin does provide features, like allowing people to create their own sites. But I'm just using multisite for a multilanguage site, so I'm creating the handful of sites upfront as the super admin.
I always found it strange that domain mapping was not core to multisite. Is it the case that the functionality is being moved in?