If you happen to figure out or know other bits of the header, feel free to add it in or tell me. Unfortunately, it probably won't completely help you find the bitrate unless you want to do some really funky stuff and just compare unknown bitrate chunks to bitrate chunks you at least know are certain bitrates (since AT3 only uses certain bitrates IIRC)
I've added some new stuff about the unknowns of the bitrate thanks to the multimedia.cx wiki while comparing your values to what they say are valid bitrates (the bitrates for at3+ are fixed).Thank you, i actually made a lot of progress, i shall update that wiki sheet with the known bitrate values so far.