Considering the information that is in the wp-config file, how secure is this file? I have password salts in place, but can someone view this file? Or to what level of difficulty can some view the contents?
This is really more of a server configuration question. By necessity,
wp-config.php must be readable by WordPress itself, but file access/security beyond that is really a matter of how your server is configured.
This could be hacked if for example a bad plugin is installed and creates flaws. This could be hacked because of the CRSF flaw that is to say a hacker utilizes an approved user account to make bad things.
You could start with this in .htaccess:
<files wp-config.php> order allow,deny deny from all </files>
But there is not an absolute protection.