Hi,
JoomSEF cannot work if there's no .htaccess file, so you should rename the htaccess.txt to .htaccess and solve the 500 error. The 500 Internal server error usually means some misconfiguration in .htaccess file, usually adding the
or removing the
helps.
You shouldn't use the ErrorDocument to handle SEF URLs from JoomSEF. Just use the default Joomla's htaccess.txt renamed to .htaccess and adjust it to meet your server's requirements.
To prevent the Artio link being displayed, you should disable JoomSEF's plugin when SEF is disabled on your website.