Links to files breaking
Site internal links should always use URIs relative to your base URL. This would avoid your specific issue. (File entities actually do use URIs relative to the configured public files folder, so that - depending on how you created your file URLs within CKEditor - clearing your caches might help already.)

However, in a mirrored multi-site environment, you can also use the sites.php file to define the base URLs and their respective target site paths. If not configured otherwise, your files folder should use the configured target sites path and all your environments will use the same files location.

Place a sites.php file in your [DRUPAL_ROOT]/sites folder (e.g. by copying and renaming the existing examples.sites.php file) and configure the $sites array keys to your requirements. Example:

$sites['site1.com'] = 'site1.com';
$sites['site1-testing.com'] = 'site1.com';