Did you restart Apache after making the changes to the php.ini file?
If you are still not seeing errors you can manually check /var/www/vhosts/<domain>/statistics/logs where <domain> is the domain name you are wanting to check the logs for.
I am sorry you ran into issues with our support team. If you wish to PM me the ticket number I will follow up about this.
Also you can edit the php.ini file and add "error_reporting = E_ALL". This will enable reporting for all PHP errors.
You can read more about that here...
At the domain registrar you will change the nameservers for secondsite.com to ns1.mainsite.com and ns2.mainsite.com.
In Plesk you will add these two NS records to secondsite.com:
secondsite.com NS ns1.mainsite.com
secondsite.com NS ns2.mainsite.com
That should allow you to use...