Good to see some "technical" discussion of ACARS.
It is my understanding from Wiki ACAR article and other sources that FMS data is usually "one way" - from ground to the aircraft. And then the crew can use a change in route or ignore it. Going air-to-ground, seems the data is up to the carrier to decide what is reported.
Am I close?
I also note that the aircraft we are looking at sent ACARS at 30 minute intervals. Is that the norm?