The one-way glass was to refute the idea that people would only watch an interview/interrogation if they were recording it. Watching without recording certainly isn't something that wasn't done often historically. The assertion that they wouldn't watch if they weren't recording doesn't stand up.
They were watching on CCTV (that's closed circuit television), which doesn't need to have a recording function but I'd bet it does.
At the time of the interrogation they were not fabricating a case (Anglo not withstanding

) and the idea that the recording would be kept secret makes no sense. If they had a recording it is almost certain that the people transcribing the statement would look and listen to the recording.
The idea that they were conspiring to frame Amanda on the 5th is absurd. They thought she was involved and they were excited that they had closed the case. The station would be abuzz with the breaking developments. Even if people weren't told specifically about the interrogation recordings, they would know that it was standard procedure to make recordings and we have a story like the one from the prison guard about Amanda. There is too much money on the table for silence.