How do I access webmail for my other mail pop accounts? What do I enter for login info in the login screens of horde or neomail?

You can access webmail via http://server_IP:2095

In login box you need to put your username (same as username for access to cpanel). It's valid for main mailbox yourlogin@yourdomain.com only. But if you have additional mail accounts (e.g. another_user@yourdomain.com you need put your full mailbox name (another_user@yourdomain.com - with 'at' and domain name. This feature is developed in cpanel in order to allow different users to create mail accounts with same name (e.g. admin@domain1.com and admin@domain2.com)