FBC 1.5

This is significantly out of date. Please upgrade to FBC 1.8.

API Error Code: 191
API Error Description: The specified URL is not owned by the application
Error Message: Invalid redirect_uri: Given URL is not allowed by the Application configuration.

The problem is exactly what the error says. You configured your Facebook account for www.example.com and tried to using FBC on example.com (this is just an example) which they are not one in the same. Facebook is very strict on domain matching.

just tried it with a FB account that did NOT exist in CB Users, and with an email that DOES exist in CB Users -

You can't do this. You can't have an email address registered twice. Nor does FBC link users based off email, it's purely based off their FB ID. If the email address for the FB account is already registered it will error out and tell them the email is already registered. This assumes your Joomla template has the message tag so you can see such messages; always test in default Joomla.