did you test it? You cant login in a wpms with your rewrite code. You can get to the login page. But after entering the password you get the 404.
It does not work with mapped domains in wpms unless you remove the root domainurl from the htaccess and replace it by a *
Pls set it up map a domain and try. It does not let you login. Mapped domains act as a root domain not as a subdomain. Especially when you also haven hidden the subdomain completely by changing that url to the mapped domain url also in the site settings of that (sub)domain,
Its not a child domain anymore.
Please test this.