You are correct that you must have a NT button programmed on a phone for night ringing. No external interface is needed for ring over page, but the system has to be in night mode.
You are also correct that not having a NT key has no effect on external paging. As far as the external paging, you seem to have verified everything down to a bad paging port. I've never had a page port go bad, but anything is possible. One thing I would still do is double check the wiring on the RCA plug for a bad connection.

A quick swap with a spare, defaulted KSU would verify your findings.