You do Not use the .htaccess file for domain mapping.
"You cannot map a domain to the main site." -- the lines you showed, from your .htaccess file, have DOMAIN_CURRENT_SITE as one of the sites you said you want to map.
I would recommend you don't use the main multi-site domain for content. Best to keep the multi-site administration using the default theme and minimal plugins; use subdomains for custom themes and plugins.
Purchase whatever type of domain name is cheapest (might be .info), to use for the multi-site administration (said another way, to be the unmapped domain of all the sites). Some domain name you wouldn't send visitors to, could even be a few random characters.
Then map your public domains ("main domain" and "second domain"), using the Domain Mapping Plugin.