Yes, metadata is stored in the database and derivatives (thumbnails and view sized images) are generated under the _data/i/ directory (_data/i/galleries/ for physical albums), original image directories are not written to. Ensure the user the web server runs as has directory traversal and file read permissions in your NAS directories, then creating a symbolic link in Piwigo's galleries/ directory should work.

In your local config (edit local/config/config.inc.php or use the LocalFiles Editor plugin) add an entry for $conf['sync_chars_regex'], look it up in include/config_default.inc.php (do not edit that file) and read the description for possible strategies and limitations. If it's only about the () characters then this should suffice:

$conf['sync_chars_regex'] = '/^[a-zA-Z0-9-_.()]+$/';