Perhaps I can help clarify these matters at least a bit. When in the Seattle and Portland areas, the Air Traffic Control transcript (which has been heavily redacted) contained only the information related to the air traffic control functions. These exchanges between the flight crew and various air traffic controllers were made on the standard ATC VHF radio frequencies. These exchanges could be heard by anyone who had a VHF receiver that could be tuned to the appropriate frequency and was within range of the transmissions by the airliner and the controllers. The controllers also had telephone conversations between each other while coordinating the controlling of the airliner. These telephone conversations would also be recorded but not necessarily embedded in the same tape as the radio transmissions.
The aircraft performance information, plus other things, was being transmitted on an ARINC VHF radio frequency, "formatted" by the receiving ARINC ground station, and then transmitted on the ARINC teletype network to various Northwest Airlines locations including Seattle and Minneapolis. In the special case of the hijacking, the ARINC VHF radio communications were also phone patched to Seattle and Minneapolis and perhaps other locations. Thus the NWA ground stations that had the phone patches would hear the transmissions from the airliner on the ARINC frequency live and then, a few minutes later, would receive the formatted teletype version of the message. The teletype version of the message would contain a time record that indicated when it was transmitted on the teletype system and this could be, and would be, a few minutes after the actual live transmission. One of the people in Seattle lists the time of the live reception of the "23 DME miles south of Portland" message as 8:18 while the teletype version was transmitted with an 8:22 time hack.
Presumably, the entire voice radio exchanges between the airliner, ARINC personnel, and NWA personnel would have been tape recorded. And the radio exchanges on the ARINC frequency could be heard by anyone with a VHF radio receiver and within range of the appropriate transmitters. However, there does not seem to have been a complete transcript prepared of the ARINC messages related to the NWA airliner.
Robert99