Regarding decoding, is there a way to listen to the datatrack in its .iso form?
I'm not sure what you mean here...
When you say "in .iso form", are you talking about a track-by-track, or a full-disc image ?
In order to do anything, you would need to start with track files - most will be simple audio (CD audio tracks), but some will be data tracks (2048-byte MODE 1). a full disc image will add all sorts of ECC error checks data which just confuse things.
Once you have a CUE/track extract, another more accurate way to find the locations of the ADPCM data, is to use the Mednafen debugger, and keep track of data loaded into ADPCM memory (and played); you would also need to keep track of which sectors are loaded from disc into memory too.
Many people might find this boring, tedious, or too much work - but it's the best way to have a full working knowledge of the system, and get accurate lists of memory locations where messages reside in the data track.