background-image and sites.php
It sounds like you're confusing hostnames with file system paths.

The array mapping in sites.php is:

Hostname =&gt; Instance Folder


So taking your example, requests for http://mysite.example.mydomain.com will use the folder /sites/mysitedev.example.mydomain.com.

In that scenario, the path /sites/mysite.example.mydomain.com/files doesn't exist, so requests for files under it will return a 404. That's to be expected.

You can always add an alias/redirect via your server config, but it might be best to just use the system as it was intended.