I'm not aware of any issues with ironport.custhelp.com. If you are unable to get it to work, you may want to file a support ticket.
The secure items message is a browser warning and should be no different then when browsing direct to an HTTPS site that has HTTP objects on it.
In IE, this may be changed by Tools -> Internet Options -> "Warn if changing between secure and not secure mode". But I'm not entirely sure. You may be able to turn this off from the warning message itself as well.